What was the basis of the formation of East Pakistan and West Pakistan at the time of the partition of India?

Common religion

In Bengal the migration was even more protracted, with people moving across a porous border. This also meant that the Bengali division produced a process of suffering that may have been less concentrated but was as agonising. Furthermore, unlike the Punjab, the exchange of population in Bengal was not near-total. Many Bengali Hindus remained in East Pakistan while many Bengali Muslims continued to live in West Bengal. Finally, Bengali Muslims (East Pakistanis) rejected Jinnah’s two-nation theory through political action, breaking away from Pakistan and creating Bangladesh in 1971-72. A common religion could not hold East and West Pakistan together.