i think with so much discussion and source-citing in the language related threads, there was an info overload. and the drawn conclusions got missed. what i am saying is not khair_ctg's opinion but plain fact.
the formal Bengali has always historically been foreign to Bengali Muslims. our native dialects back then and now (Musalman Bengali) was not standardized, because we used to use Persian or Urdu as a standardized refined language. the occasional instances that Bangla was written in Persian-Arabic script was not very widespread. for example, we do not use chatgaiya as a written language because it was not developed as such. whereas the Muslims contributed to the development of Farsi and Urdu, the Hindus developed Bengali especially in the colonial period. that is why formal Bengali became a Hindu language.
during colonial period even though Muslims would be adept at Urdu and some, at Farsi, they started to learn the formal Bangla or the Hindu Bangla, because it was a Hindu dominated society in Bengal. the descendants of a small number of those very Muslims became the leftist/communists we modern Bangladeshis know as. this invasion by Bengali leftists/communists, also backed by Calcutta babus, using formal Bengali has to do with a broader scheme of getting rid of any Muslim aristocracy and any associated refined culture. as such the hostility shown towards Urdu for example by some Bengali leftists was not really part of their hostility towards former West Pakistan, but a hostility towards the Muslim heritage of East Bengal/EP/BD. this has obviously devastated Bengal's Muslim society, made us isolated and made us pawns of the Hindus according to history. and sorry i don't see any silver lining here