Good point. American religiosity is, for the most part, not about spirituality, but about nationalism. In a country that is a melting pot of peoples from all around the world, the old fashioned nationalism appealing to common ethnic ancestry (Aryan or otherwise) would have rather limited appeal. A "Christian" identity (complete with a "Christian" flag which is a historical nonsense), otoh, crosses the ethnic lines, but at the same time excludes all "undesirables" such as liberals, Jews, homosexuals, feminists, and a great deal of people of color. It is thus a perfect nationalist ideology in a multi-ethnic state.
And, indeed, it is a true "god's country" outside college campuses and major urban centers. Heil Jesus!
Wojtek