It has not balkanised because, there isn't any superpower supporting teh rebels and minorities are too few in number compared to majority hindu.
The hypothesis of possible balkanisation of India is primarily based on the separate identities of language and regional cultures. Accepting that Hindus are in majority, the cultural differences among different states were/are supposed to be the faultlines in India and not religions, which are generally evenly spread across the country. By that measure, Hindi speaking and not Hindus are the majority. But the others are not insignificant minorities.
India has far too many different religions, regions, cultures and ideas. It also has (more than) its share of adversaries and opposing ideas. But let's face it, the abstract idea of India seems to be resilliant enough to withstand and move forward.
Kuchh baat hai ki hasti mitati nahi hamari....