Thats because the Uyghur issue is rooted in ethnic separatism, not religion, and there are millions of other Muslims in China, Hui Muslims outnumber Uyghurs and they aren't separatist. China has Hui, Dongxiang, Turkic Salar, and Bonan Muslims who are fanatically religious and more devout than Uyghurs and have no problems with them, they sre used to promote ties and investment with Muslim states. In fact Uyghurs are the least religious Muslims in China. If Iran supports ethnic separatists in another country, it will get massive blowback over Kurdish, Baluch, Arab, and Azeri ethnic separtists in its own country. Iran's government is based on Shia Islamism, not principles of ethnic nationalism that every ethnicity gets its own nation.
The Soviet Union in fact did the same thing to Iran and China in the 1940s with ethnic separatists. Iran does not want this to happen again. If Iran supports ethnic separatists in another country, that country will be hosting Arabs, Kurds, Baluchis, etc. and supplying them with military and political support.
Iran crisis of 1946 - Wikipedia, the free encyclopedia
Azerbaijan People's Government - Wikipedia, the free encyclopedia
Republic of Mahabad - Wikipedia, the free encyclopedia
Ili Rebellion - Wikipedia, the free encyclopedia
Second East Turkestan Republic - Wikipedia, the free encyclopedia
By the way, your media has produced seriously false libel on this issue over migration. There are separatist movements which have nothing to do with demographics (Chechens made up the majority of people in Chechnya when the war occured, it wasn't Russian migration which set it off, Kurds also make up a large majority of the parts of Turkey and Iraq they live in). Its poverty, neglect, lack of economic opportunities, and people fanning extreme nationalism which can cause separatism.
The Truth about Uyghur demographics and Han immigration in Xinjiang