I recently touched on this topic in an earlier comment: https://np.reddit.com/r/AskHistorians/comments/7ou8pa/despite_the_presence_of_large_islamic_empires/dscf5cb/

Basically, eastern Bengal (especially the southern and eastern portions of what is now Bangladesh) was an economically underdeveloped region up until the period of Muslim domination. Pre-Islamic dynasties like the Palas and Senas had their centers of power in western Bengal, such as at Gauda/Lakhnauti and Nabadwipa, and not in the backwaters of eastern Bengal. It was under Islamic rule that the eastern parts of Bengal were intensely settled and cultivated, and because of the supervising role that Muslims played in this process, they became the local elites of the newly-civilized society in eastern Bengal and were able to establish their influence at a grassroots level, absorbing many Bengali colonists into the Islamic fold. Western Bengal, by contrast, already had a well-established social and economic order that was long under the domination of a deeply conservative Hindu aristocracy, and so Islamic influence in western Bengal was much weaker at the local level. A similar dynamic also explains why frontier regions in modern-day Pakistan became thoroughly Islamized while the long-settled and highly-stratified regions of UP and Bihar remained strong bastions of Hinduism.

If you are interested in learning more about this topic, I recommend the book The Rise of Islam and the Bengal Frontier, 1204-1760 by Richard Eaton. His work offers the most important contribution to understanding Islamization in eastern Bengal, with implications for other parts of India as well.