Here's another analogy that maybe explains the staying power but also liability of theism versus humanism.

Religion : Humanism :: a vow of abstinence : the value of temperance

It's a lot easier to get a whole lot of people on the same page with black and white concepts- even when you have to somehow negotiate the mental gymnastics of embracing those concepts as both literal realities and allegories simultaneously, than it is to get people to form a community around abstract values that they sort of have to figure out how to interpret on their own.