Why are Americans so religious?

Keep in mind that, while America is one of the most religious developed countries in the world, a significant number (up to 20%) of Americans are either atheists, agnostics, or follow no organized religion.

America was FOUNDED on concern for human rights. On paper at least, America has been welcoming to people of all faiths, and there exists in the US a "marketplace of faith", where Americans can choose the religion (or lack of religion) they most closely identify with. Religious movements in the United States have often been populist in nature, with high levels of participation by ordinary people. This populism has allowed ordinary Americans to mold religious establishments and beliefs over time, where European traditions of aristocratic control have stagnated religion there, leaving more people disenchanted.