Just speculation without any data to back it up: * A selling point of America in the beginning was freedom of religion. * A lot of people were killed off in WW2

Just speculation without any data to back it up: * A selling point of America in the beginning was freedom of religion. * A lot of people were killed off in WW2 because of their religion, and I wouldn't be surprised if many lost their faith because of the deeds committed under these times. (made it hard to believe in a benevolent God with a plan) * A big catchphrase of the socialist movement was "Religion is the opiate of the people".