history : Why-didnt-complete-population-exchange-take-place-during-the-partition-of-India
Answer by Murtaza Aliakbar:
It was unwise to partition the British Empire on basis of religion in the first place.
Migrations did happen, some of it were forced. And if you dig up history books, you'd find that the Muslims of Konkan, Trivandrum, Osmania, and even Punjab weren't that keen on creation of a new nation state. It was primarily the Muslim elite in United Provinces and Bengal (which was the first victim of British Raj's 'divide and conquer' policy) that favoured it.
Various conspiracy theories are abound on how quickly Pakistan went from being an idea to actually being in-existence. The very first thing Jinnah said on landing in Karachi, after partition was agreed, was that he never for once believed Pakistan would become a reality in his lifetime. One conspiracy theory is that the Britishers were wary of leaving behind region in a stable state. With partition in place they ensured the region would continue in conflict and could be a battle ground for the west for its war/geo-political games. This was very a common tactic of imperialist nations to stay on the top by forcing others around them deep into the rubble (they broke up the Muslim Empire in mid-east, and in Northern Africa to the same effect as well).
Being a single nation would have made India a very powerful force in the region. Imagine a country united of size as big as India (+Pak +Bangladesh) and population as big as India's (+Pak +Bangladesh). Direct borders with central Asian countries which is a paradise as far as availability of natural resources is concerned. Two major river systems following through the country in Indus and Ganges. No petty wars and no Kashmir issue to keep the army busy. No insurgency. The world as we know it would have been very different.
Migration never made sense, as you had to leave behind all your property, your friends, your home for a strange new land with added excitment of being killed/looted/raped/die on the way. But still, given the odds, it was the largest recorded migration in History. Partition drained the country of 60% of its Muslim population, which in itself is a shockingly huge number. In a sense, despite not much support (or help from the govt), and despite the conditions not being favourable (the Indian summer, the long commute) a lot of migration did happen.
Why didn't it happen 100%, as in why didn't India get rid of all its Muslims… Well, that's because the leaders were *never* in favour of the partition. The Congress was unanimous in their stance that dividing the region on basis of religion was a mistake. India, as Congress envisioned, was everything that Pakistan couldn't be– a secular nation. And for that reason, there's record of Muslims that migrated from Pakistan to India as well, although, small in number.
It was unwise to break up the nation. It would have been even more idiotic to have pushed the nation to its limits by committing to cleanse the region of Muslims. Because the folly in that logic is clear… Where would you put other minorities? Give them their own countries as well? Would they feel secure? How would you stop other such separatist movements that such a mass exodus would trigger? What was the point then of fighting the for independence the Gandhi way? What would you do with politcally powerful Muslims still part of the Congress setup? How would you prevent the situation from exploding into nationwide rioting and looting?
The priority in 1947 was nation building. Creation of Pakistan could have destabilised the nation further. It didn't. Thanks to Sardar Patel. So given the fragile state of the country then, it was perhaps a no-brainer. And perhaps the next thing the freedom fighters would have wanted to priortise would have been to disprove the idea on basis of which Pakistan was created. And one wouldn't have been able to achieve that with a 100% Hindu country.