Considering how things panned out in 1971, people would wonder why Bangladesh used to be a part of Pakistan in the first place. However, elite Bengali speaking Muslims were the bedrock of the Pakistan movement and they overwhelmingly voted for the Muslim league in the 1946 provincial election, which many would claim to have been the election that made Pakistan, winning 113 out 119 of the seats reserved for Muslims in united Bengal.

The real question in 1946-46 was whether Bengal was going to be divided or undivided, divided in India, or divided in Pakistan. Some leaders like Suhrawady and Sarat Chandra Bose weren't keen on seeing Bengal divided again, however, most League leaders and representatives and leaders from East Bengal, Indian National Congress, Hindu Mahasabha, and the Communist Party of India did not sympathize with their sentiments. By 20 June 1947 the decision to partition Bengal along religious lines was decided. Few weeks later a commission led by Cyril Radcliffe drew a line that divides Bengal even to this day.

References

The election that created Pakistan

Peacocks at Sunset - On the partition

Colonialism, politics of language and partition of Bengal

Chapter on Modern Bengal of Land of Two Rivers: A History of Bengal from the Mahabharata to Mujib by Nitish Sengupta