I'll tell you why: social media.
Since Mark Zuckerberg founded Facebook in 2005, Americans have been getting fed a continuous drumbeat of how much their country sucks and how the Nordic countries are the ones to be lauded and emulated. Of course, Sweden, once the cradle of Lutheranism, is now one of the least religious countries in the world. If we only abandon our faith, we could be like them!
For all this talk of Americans wanting more religion in public life, church attendance is still way down.