The SBC has a very stodgy, buttoned-up image and view on personal behavior. For instance, more traditionalist members oppose consuming alcohol.
That doesn't really jibe with the libertine tendencies of modern American conservatism. The Republican Party is led by Mr. Pussy Grabber. And, no alcohol? That sounds like something Muslims do! Can't have that!
No surprise the marginal members would rather go to a non-denominational megachurch that tells them making a bunch of money and buying a bunch of shiny, flashy consumer products means they're fulfilling God's commands.
A lot of Evangelical things are Muslim things.