My guess is that it's correlated with urbanisation. The USA still has lots of people living in more rural areas, and those tend to be more religious. Cities g

My guess is that it's correlated with urbanisation. The USA still has lots of people living in more rural areas, and those tend to be more religious. Cities give people more options and places to hide - it's very hard for a church to exert that much influence, compared to a small town. So people can split off and do their own thing.