Good question. A short answer would be that PPP even from early 1970's was a Sindhi ethnic party. Mumtaz Bhutto's equating Sindhi language with Urdu in (1972?) caused ethnic riots in Urban Sindh. I remember the graffitis in mid 70's Karachi: "Let's make Karachi a Province". "Let bring back Urdu as our language." That was Karachi's reaction to Mumtaz Bhutto. But..also the Mohajirs of Karachi/Urban Sindh were, while more liberal than other Pakistanis, still thought 'Islam' as the primary identity. So they voted for JUP (Noorani) and Jamaat e Islami in the 1970/1977 elections. Prior to that, they voted for Fatima Jinnah. There was no MQM.
Anyway, there were/are no Pakistanis more devoted to Pakistan than the migrants from India. We had burned our boats to migrate from India. Urdu was/should be THE National, unifying language of Pakistan. The Bengalis of E. Pakistan thought otherwise. The Sindhis thought/still think so that their's is the greatest. I don't think either Sindhi or Bengali language should vanish: But to build a 'nation', you DO NEED some unifying language. Urdu was Quad e Azam's choice. It should be Pakistan's choice.