"Religion is much more important to Americans than to people living in other wealthy nations. Six-in-ten (59%) people in the U.S. say religion plays a very important role in their lives. This is roughly twice the percentage of self-avowed religious people in Canada (30%), and an even higher proportion when compared with Japan and Western Europe. Americans’ views are closer to people in developing nations than to the publics of developed nations."
http://www.pewglobal.org/2002/12/19/among-wealthy-nations/How did this happen? I'm not saying it's wrong, but it definitely is unusual for a wealthy country to be less religious.
I think a possible reason, was that because of the Native Americans here, the first European immigrants felt a need to Christianize them, and so kept up discussion of the faith. They did the same with the slaves, in making them Christian. I also think this is why Latin America has a high religious rate as well.