I can identify two reasons:
1- The immigration of people from other areas diminished the importance of Punjabi language and culture in daily life.
2- The constant charges of misappropriation and extortion made people feel detached from their culture.
I'd add a third to it too :
3- The Punjabi Muslims never developed a sense of an ethno-linguistic nationalism as evidenced by his lack of talk of 'my land, my people, my language' kinda discourse when immigrants like myself were coming to Punjab from nearby regions over the centuries nor did he feel the need to galvanize himself into a Nation under One Ruler or anything of the sort and build an Empire of his own because he never was into that sort of thing nor was he pushed into developing that sense of 'Me under Trouble and therefore I should have my own piece of Land' like the Sikhs were after the Mughals had their go at them.
I think that lack of 'nasal parasti or qaum parasti', in Urdu, is the reason why Punjabis were never fixated on learning or preserving their language and culture that much and that coupled with points 1 and 2 brought upon a desire to adopt a more pan-Pakistani identity where Urdu was elevated as a more worthy language to learn because it represented the Federation !
In a way I'm sad that this happened because Punjabi is such a beautiful language and the culture of Punjab is cherishable - It should've been preserved; however I'm also glad in the sense that maybe if the Punjabis had taken the route that fills one's head with the kind of ethno-linguistic nationalism that demarcates between people where the original vs the local kinda crap finds mention - I, an ethnic Kashmiri, along with tens of thousands like me would probably not have the sense of ownership that we have to Punjab because no one shoved the Punjabi language or the Punjabi culture down our throats.....we learned it just because our friends and our family around us were speaking it and practicing it and that kinda made it feel like our language and our culture and over the generations it really did become our language and our culture.
Take the example of PML N - its the largest party in Punjab and its full of ethnic Kashmiri - this would not happen in Baluchistan, in Sindh, in KP or anywhere else in the region and yet not even their most ardent detractors here in Punjab have thrown a racial slur at them or asked why are Punjabis being ruled by a bunch of Immigrants from the North ?
I think somethings are worth praising - Punjab's Pluralism is one of those things & thats why, within Pakistan, I take great pride in calling myself a Punjabi !
P.S The reason people stare at you when you speak Punjabi in a suit-piece is because you speak Gulabi Punjabi !
Btw none of these people are celebrities. Just random people through google
