Said the guy whose life mission seems to be to convert a non-profit into a for-profit entity.
I thought it was because everyone was accepted, technically equal, and sins were seen as something inherent and forgivable (at least with Christianity) whereas paganism and polythiesms can tend towards rewarding those with greater resources (who can afford to sacrifice an entire bull every religious cycle), thereby creating a form of religious inequality. At least that was one of the somewhat compelling arguments I heard that described the spread of Christianity throughout the Roman Empire.