IR : Why-didnt-Bangladesh-choose-to-join-India-after-the-Indo-Pak-War-of-1971
Answer by Balaji Viswanathan:
Once the bullet leaves the barrel it is often hard to put it back. East Pakistan [before it was renamed Bangladesh] spent years fighting India. The worst of pre-partition violence happened in East Pakistan. Then after partition, it was a part of a country that fought 2 wars with India.
Given all that history, there was no overwhelming interest to rejoin India. In fact, it is very unlikely for nations to be reunited. In the rare cases it happened – Vietnam & Germany – the original partition was pushed by the outsiders and artificial. Thus, once the outside influence was gone they reunited. In India's case, the partition had a deeper support. India was also way different and larger than Bangladesh.
And thus there was not even a question of Bangladesh rejoining India.