Because we're geographically mobile and ethnically diverse, says Brink Lindsey. Because our families are stronger than Europe's, says Mary Eberstadt. My own preferred explanation - which is doubtless a small part of the pantomime - is theological rather than sociological: Christianity has thrived in the United States by adapting its theology to the habits and mores of the American people, in a way that religion in Europe hasn't managed to do. America is an Emersonian country, and its religious innovators have invented an Emersonian form of Christianity - which some might suggest isn't Christianity at all, of course - that's nicely tailored to the broader culture in which it swims. Call it gnosticism, or Moral Therapeutic Deism, or just plain Americanism - it means Elaine Pagels and Karen Armstrong for highbrow audiences and T.D. Jakes and Joyce Meyer for the masses, and it works.
If Christianity in America meant the Christianity of Benedict XVI - or even the Christianity of C.S. Lewis, for that matter - I bet that about 15 percent of the country would be practicing believers. But you don't get Benedict or even Lewis from most pulpits; you get socially-conservative Emersonianism in Red America and socially-liberal Emersonianism in Blue America. This wouldn't fly in the European cultural context, but maybe there's a form of organized religion that would - its theology just hasn't been invented yet.