I would say unlike Russia in the 1990's and early 2000's, there are no attempts on China's part to test out hypothetical prospects of integration or strategic partnership with a USA-led western block. China is firmly set for an adversarial course against Washington. The gigantic BRI project is basically Beijing's expected roadmap towards pole position in the framework of this very rivalry.
Therefore explanations for Beijing's policies ought to be sought elsewhere. Comments below could offer a partial insight into the question.
Claims by western analysts on a subject such as Iran-China relations, or Iran-Russia relations for that matter, should be met with reservations. Time will tell. What we can be sure of though, is that western sources will start issuing pessimistic assessments on Iran's relations with partners even as groundbreaking steps are effectively taken. Read on if you will for further details.
What Iran can do is to grant Chinese investment as well as the completion of international transport corridors the priority status they call for, if it's not been done already by the Ra'isi cabinet.
Ergo, and here we'll touch upon a decisive ramification of the problematic at hand, ditch once and for all the belief that oil should and can continue to fulfill the significant role it used to play for the past hundred years in the Iranian economy. And along with that, bury once and for all any remaining hopes of clinching some sort of a "deal" with the west whereby the latter would lift existing obstacles over the export of Iranian crude. The exponential rise of American domestic oil production coupled with the power of associated lobbies (to whom Iran would represent an unwanted competitor) mean that Iranian oil is never going to make a resounding comeback on international markets. It's never going to happen, simple as that.
Once this realization sinks in with the Iranian political class and the necessary conclusions are drawn, more efficient steps are bound to be taken in regard to Chinese investment. What substitute is there for oil as a guarantor of long term economic stability from Iran's perspective? Ultimately it ought to be the non-oil sector that takes over this role, industries and agriculture included.
But until this ambitious goal is fully met, and it must be pursued at full force, some parallel resource has to be relied upon. Not least because a transition of this kind never amounts to a cakewalk for a nation whose prime economic focus has seldom been on the industries. This involves much more than purely economic reforms, namely some upstream reshaping of deeply ingrained cultural mindsets.
So the answer lies in Iran's historic role as a global transport corridor stretching back to the Silk Road era and further yet, as the single most concentrated hub of strategic transport routes across the Eurasian continent. In terms of lending itself to the establishment of transport corridors, no country is blessed with a geographic location as beneficial as Iran's. This natural asset, whose massive prospective windfall becomes apparent in light of the staggering amounts China is planning to invest into its One Belt One Road initiative, must be made fruitful. Closely related to such corridors, Iran's trade sector blossomed over centuries. Remember how back in the day, Iranian traders played a key role in expanding Islam towards South and Southeast Asia.
The previously mentioned thinking of old, more or less beholden as it remains to the notion of a relatively large oil sector, takes time to vanish. It has nonetheless been an impediment to the exploitation of transit route opportunities to their full potential. Said potential according to some analysts is so vast that it could safeguard Iran's economic safety for the next hundred years. This is not merely a figure of speech: serious actors plan for different chronological horizons, including the very long one. China's plans are for the next hundred years. Hence why it would be misplaced to expect all too many erratic, sudden revisions of strategy or even tactics from PRC.
What remains to be examined at this point is how the different political camps in Iran have been tackling the issue and what their respective positions have been. Here it appears clearly that the Rohani administration literally squandered at least two golden opportunities (in 1394 and 1398) to seal an effective, large scale bilateral economic cooperation program with China. And it did so out of political and ideological conviction, seeing how it forbade itself to conceive of any solution for Iran outside the chimeric, counter-revolutionary liberal dream of normalization with the West. A third opportunity was lost in 1388 when radical liberals shouting "down with Russia, down with China" in the context of the failed "Green Movement" fitna, effectively prevented the second Ahmadinejad government from going ahead with the signature of major contracts with Beijing for fear of renewed unrest triggered by the liberal crowd.
What this resulted in, was the truly insulting reception of the Chinese president in the wake of the JCPoA's conclusion. Indeed, not only were ceremonial formalities kept to a minimum. Not only did Rohani convey to the Chinese that "if we sought close cooperation with you, we would not have spent years hammering out the JCPoA with western governments". But moreover, Rohani left Tehran for Rome while Xi was still in Iran for planned talks. And, upon exiting the airplane surrounded by Italian officials, Rohani exhibited one of those disgraceful laughters. This is on top of reports that Xi was given a room deprived of a water closet of its own during his stay in Tehran, and that he was thus compelled to use a shared one on the hotel floor. Mind you, Xi was the first major head of state to visit Iran after the JCPoA was concluded, and he had definitely come with a concrete offer which he put on the table to massively boost economic ties with and investment in Iran. His enthusiasm was shattered by the moderate administration's unbelievable conduct.
Now this begs the question, why should China be expected to do Iran any favors when their president is treated in such a manner during an official state visit which Beijing hoped would form the starting point of precisely the sort of enhanced bilateral cooperation we're envisaging? Up to this point it was squarely on the moderate Rohani administration and nobody else - other than NATO regimes and their clients in Riyadh which used their well funded media apparati (the Saudi International broadcasting network has been deeply involved in this) to keep tarnishing China's image with Iranian audiences exposed to their incessant propaganda, and to spread blatant falsehoods about the Islamic Republic and in particular the IRGC and revolutionary / principlist factions supposedly "selling out" the nation's wealth, even the whole island of Kish to the Chinese!
The drivel was echoed by the liberal (reformist and moderate) press as well as by a figure like Ahmadinejad. Of course there is no inkling of truth in such ludicrous stories, yet their proliferation makes it all the more difficult for Iranian decision makers to compensate for the previous administration's blunders on China relations, given the impact of said propaganda upon certain segments of Iranian public opinion. Enter seyyed Ra'isi's election to the presidency less than two years ago.
From what I could gather so far, it's too early to pass judgement as to whether or not the current cabinet is giving the dossier of Iran's bilateral relationship with China the necessary attention. This being the kind of topic which requires a special appointee to oversee the development of ties, and to work in tandem with specialized sub-commissions. This appointee must not be given other tasks to manage and they must be a high ranking current or former official, such as Ali Larijani who was fulfilling this role before. Above all, in preparation for Iran's economic futur the administration must be driven by firm political will to pursue alternatives to the JCPoA as well as to oil exports.
This said, there is an undeniable contrast in the geostrategic outlooks of the Rohani and Ra'isi administrations, which should not be lost on Beijing. If the Chinese consider both to be equally uninterested in propulsing Iran-China relations to the next level, they'll be committing a sizeable mistake in their calculations. In other words, taking it out on the current government because of the policies of the previous one would be particularly inappropriate in view of not just Iran's but of China's own interests as well.
Lastly, what about the Saudi and PGCC factor in all of this? One thing to note in this regard is that Beijing's interest in the PGCC is not primarily related to the OBOR / BRI, whose land component is the most important one. Their mutual partnership from China's standpoint revolves mostly around energy. For China, relations with Iran and with the PGCC in essence do not constitute a zero sum game, the recent controversial joint statement notwithstanding.
Another much less talked about parameter might be the zionist and Haifan Bahai one. Both these entities have non-negligible relations with and presence in China. But their hypothetical impact on China's policy towards Iran to my knowledge is yet to yield sufficient in depth research.
This leads me to the concluding point of the present discussion. As correctly highlighted by the source shared below, since the OBOR is pivotal to China's fundamental strategic planning, its relevance is reflected in scores of geopolitical events, crises, conflicts, and other international phenomena throughout the Eurasian landmass. By way of consequence observers are invited to pay attention to easily overlooked, OBOR-related causalities and implications in those mentioned occurrences.
As an example, there's another reading or better said a second, parallel angle to the Zangezur issue which none of us had talked about if I'm not mistaken: beyond the problematic of the "ethno"-separatist anti-Iranian agenda which the regime in Baku is regularly activating under the influence of Tel Aviv, which could pose a threat to Iran's territorial integrity, think for a second about what an occupation of the Zangezur area and contiguity between the Republic of Azarbaijan's two disconnected territorial portions could imply in terms of the New Silk Road project. Namely, a more direct railway connection between the port of Baku and Turkey, in lieu of the route from Baku to Turkey via Georgia. This would be in the framework of an alternative to the original OBOR corridor traversing northern Iran from East to West.
Nonetheless, the proposal remains inferior to the Iranian transit route, because it would involve crossing the Caspian by ship whereas the path through Iran enables direct and non-stop rail transportation from Xinjiang all the way to Europe. This is beside Turkey's NATO membership which Beijing might view as a latent risk factor of sorts. At any rate Iran enjoys the geographical position most conducive to the development of transport corridors. For Iran is at a crossroads, where the East-West BRI corridor would intersect with the International North South Transport Corridor (INSTC).
But analyze last year's turmoil in Kazakhstan with China's OBOR in mind. Same with the situation in Afghanistan. Even the conflict in Ukraine itself. This might also be part of the explanation behind China's apparent timidity in assisting Russia in its war effort. China needs to keep her options open with Eastern European countries including Ukraine. Its master plan for overtaking the USA hinges upon the success of the OBOR.
The tense situation along the borders between Belarus, Russia on the one hand and Ukraine, EU member states on the other only adds to the importance of Iran as a transport hub between China and Europe. Under multiple aspects, there is no satisfactory substitute to the Iranian route for China. It's up to the Iran government to make good use of this opportunity, and up to the Chinese not to miscalculate.
We can go a step further and ponder whether the geographic location of Iran's Sistan-Baluchestan province and Iran's Kurdish-speaking areas on the East-West corridor or close to it, is really unrelated to the fact that both these regions saw relatively higher levels of disturbances during recent riots.
The above analysis was based to a considerable extent on the following presentation by Jedaal (as perceived through my personal lens):
To gain access to a more complete picture of what's going on, reading or listening to comprehensive analyses is the only option. And western sources are best avoided. So to those interested in the topic of Iran-China relations, I'd recommend listening to the above. It will answer your questions to a much better extent and far more accurately than "Twitter" one-liners and publications from western-controlled mainstream media or think tanks.