1. American churches have been, to a large degree, gutted by good old fashioned American pragmatism. We've become preoccupied with being "relevant" and "efficient" at the expense of holding fast to the theological depth of our biblically based traditions. Megachurches in particular are guilty of this -- which in part explains why they become megachurches, for relevance and efficiency sell well to baby boomers.
2. Evangelicals "have identified their movement with the culture war and with political conservatism." Spencer notes that "we fell for the trap of believing in a cause more than a faith."
Manifesting typical Constantinian triumphalism, many conservative American Christians naively thought we could transform American society in a "Christian" direction by acquiring political power to enforce our (self-proclaimed) superior views on selected topics (especially abortion, gay marriage, creationism in schools and stem cell research) on the broader culture. It has not gone well, to say the least.
After 40 years of intense political involvement, Evangelicals have little positive to show for their efforts. To the contrary, we've arguably only succeeded in getting multitudes of non-Christian (or simply non-Evangelicals) to disdain us and the "Good News" message we're supposed to bring. Now that the political parties and positions Evangelicals largely identified with have fallen on hard times, Evangelicals have, to a significant extend, fallen with them.