Mickey Maudlin, who is VP for Bible Publishing at HarperOne, believes it has something to do with the fact that evangelicals have done a better job of accommodating to consumer culture than mainline Protestants and Catholics. He writes:
Because the most important agent in this world is the individual
consumer, and because of the sheer size of this demographic, books,
music and programs are marketed to these individuals, which has allowed
for the rise of mega-churches (guaranteed quality programming), a
network of Christian bookstores and a panoply of media offerings (TV,
radio, websites, DVDs, etc.) targeted to these believers. So when an
unknown author catches on in some circles -- such as happened with Sarah
Young's devotional "Jesus Calling" -- there is a system in place to
respond (Young's book has sold more than 2 million copies). There are a
variety of ways to market effectively to their audience. Yes, those
bestsellers break out into the general market, rising in rankings on
Amazon and sold in stacks at Barnes & Noble, but often half the
sales of these blockbusters are from specifically evangelical
distribution channels. This is a huge advantage.
Evangelical print culture in the early nineteenth century was popular for the same reasons. I seem to remember Nathan Hatch having something say about this in The Democratization of American Christianity.