I think a lot of it boils down to poverty and literacy. Religious fundamentalism is largely a pastime of the poor, and as the US has become more affluent religious fervor has faded.
> Other belief system's populations die out due to lack of breeders, so it is just survivorship bias.
Breeding isn't a sure-fire way to produce more believers, either, and I say this a a non-religious person who was raised in a very religious family, married to another non-religious person who was raised in a very religious family.
There are many correlations for the last half century to try to latch onto, but there is a historical reality that continues to this day: they will outbreed you.
> Other belief system's populations die out due to lack of breeders, so it is just survivorship bias.
Breeding isn't a sure-fire way to produce more believers, either, and I say this a a non-religious person who was raised in a very religious family, married to another non-religious person who was raised in a very religious family.