I think you answered your question yourself Rethink. Part of it is in the doctrine they follow. Another part has to do with religion being the administrative power back in those days. Churches had the authority to collect taxes, and how do you increase your revenue? By increasing your member base.

Contrary to this, Hinduism has been a secluded religion where only people born within the religion normally practice the religion. I am not even sure if there is a formal method to convert one self to hinduism, like christening somebody or converting to Judaism etc.

Guess what is the difference between a cult and religion? Number of people following it.