It's basic demographics, folks. Stetzer:
Mainline Protestants, which has been the tradition of several U.S. presidents, aren’t “multiplying” with children as rapidly as evangelicals or others of differing faiths. And geography matters. Places where Protestants live are now in socio-economic decline, and parts of the country like the Sun Belt are become more evangelical with every passing winter.
Here's Stetzer's data: