@Ryan Hellyer - There are about 400,000 churches in the U.S. alone and WordPress powers an insane percentage of new websites. Add to that the fact that building a WordPress site is much more economical than most other solutions and you've got a pretty good market for church themes.

There's an established industry of proprietary hosted church CMS solutions with setup fees of $200 - $1000 and monthly fees of $20 - $60. They're economical alternatives to a fully custom site but with WordPress the cost is more like $100 once then $10/mo for hosting. That's an attractive difference, especially for small and new churches.

I've counted about 50 church themes out there at this point. Like @Okay Marketing hinted at, two years ago that might have been less than 10 in all.