U are right. Every community has divisions. But minority (muslims, christians) have a centralized control in their religion (like pope, maulanas etc.,) which unite them to vote for a group en mass... Ok if there is too large a division, why don't politicians call Dalit muslims, FC Muslims etc., and ask votes from specific sections.
Ok. Do any politician ask vote specifically from Shia muslims by putting a shia candidate? During elections, did Media hightlight any caste factors among muslims for a candidate's winnability??
Hindus are socially divided for centuries and that is the main problem. Politicians have enlarged this divide further.