"Forty or 50 years ago, it was almost a form of deviance not to be religious," he said. "When you take away that external form of motivation, people either drop away or they find their own kind of motivation."
I think that is probably the key thing-peer pressure! The more permissible something is, the more it catches on. I think that the general culture in the US considers it admirable to "do your own thing" in regard to spirituality and blending different philosophies seems popular too. Kind of a spiritual buffet approach.
Steph