Bengali people are fairly secular, so why are we still divided into West Bengal and Bangladesh due to religion?

Who has divided Bengalis ? Try to find from History .It is ist British tried and then Muslim Leauge lead by Jinna and Alibordi and finally by unfortunate partisan in 1947 and now by fundamentalists force of Bangla desh who has forgotten the contribution by India and Late PM Indira Gandhi who created Bangladesh out of Pakistan .I feel shock and bewildered when I see once a secular Bangladesh turned to a typical muslim country in line with Pakistan .