ndia is interesting in that in it we can see both of the primary vehicles through which Islam spread. This also explains why the Muslim majority areas are so...disjointed. The areas in the West came to be Muslim majority the standard way...through conquest and subsequent political domination. The Sindh and Multan regions first fell to Muslim rule in the early 700's under an Umayyad army led by one Muhammad Qasim.
The East however became Islamic at the hands of not warriors, but travelers. Both traveling merchants & traveling mystics AKA the Sufis. Trade throughout the Indian Ocean was pursued by Arab merchants since long before Islam, but the rise of the Caliphate proliferated this greatly as more and more ports became...friendly. Just as Indonesia and Malaysia and other Islands in SE Asia were exposed to Islam via merchants, so too was Bengal which since ancient times had been a key trade node.
Starting in 14th century, a bunch of Sufis from Turkey/Iraq/Iran decided to leave the familiar lands behind and venture into the eastern fringes of India. See, a lot had happened in the 700 years since the conquest of Sindh/Multan. Namely, the Delhi Sultanate had been established almost exactly a century prior, bringing the North Indian heartlands into Dar-ul-Islam. This rendered what we know today as Bengal the land beyond the farthest reaches of Islam. Which is precisely what made it so attractive to the Sufis...those hippies. They decided to leave everything behind and travel to these fringe areas to spread the good word, eventually establishing a presence in the region. Over the next few centuries, they successfully converted most of the frontier.
This is a very general overview of the course of these events. But it raises an interesting question which I don't have the answer to. That the western regions converted in totality makes sense, they were under Muslim domination since the early 700's. So 500 years prior to the Delhi Sultanate, and 700 prior to the Sufis. That the central plains didn't also kind of makes sense, as it was under Muslim rule for a much shorter period of time than other Muslim majority lands such as Sindh/Persia/Syria/Egypt etc which all took toughly 7/8 centuries for total conversion. The question is, WTF did these Sufis do that convinced Eastern Indians to convert so quickly and thoroughly? I mean, sure the entire Bengal region would come to be under the rule of Delhi and then the Mughals..but why did they convert so handily and quickly whereas the Indian heartlands did not. Considering the fact that the British Established their rule in Bengal in the late 1700's, this only made for about 400 and some odd years of Muslim rule. Thanks to the extremely patchwork records from this period, the world may never know.