For debate sake I will share my views on this a little further. 3 factors played a major role in destroying the loyalty of muslims left there towards Pakistan.
1) Muslims left there were directed to an alternate sphere of influence. They were imposed with an alien Hindustani culture and ideals. They did not grow up watching PTV patriotic dramas. This made the younger generation forget what our country Pakistan (as it was made for muslims left there too) meant. I mean clearly I ask the muslims left there what happens when India purges you as in Babri and Gujarat riots. The only place we will be able to escape to is Pakistan and thats exactly what our family did. My point is that having another sphere of influence and being subjected to Indian propaganda made muslims keen vassals repeating whatever the Hindutvas told them. I would ask Indians to ask any muslim what they think of Modi... he is a killer of thousands. But then there is this new phenomenon of slave muslims shaking hands with the same guy. Recently these same handshakers with Hindutvas have become representatives of muslims left there rather than Jinnah or Liaquat Ali Khan.
2) We migrants or mahajirs (I hate that term but due to lack of another am using this) began treating our muslims relatives left there as different. We always say for our travelling khalas or mamoos "you are going to India". we never say you are going to Lucknow-which is supposed to be part of Pakistan. In my family there has been an argument over Lucknow and Karachi even though the Muslim League was born in Lucknow. Simple thing is bhai, whether Lucknow is good or Karachi both are supposed to be treated as a part of Pakistan.
We should have treated muslims left there as a part and parcel of Pakistan. Many could be an asset in times of war with India. But our foreign policy has been one huge mess.
3) The break up of Pakistan and the establishment of a sovereign Bengal state. It proved that pakistan was breakable and also that muslims cannot live peacefully in one country, even when divided from India. I sympathize with this sentiment but at the same time know that Pakistan is the only hope for us-as it was for my parents and it received support from my grandparents