If they don't play German folk music on German radio, as alanabit says, it's not too surprising if it never turns up on the BBC.

I'm sure there must be some great German folk music, but I've never managed to find it anywhere online.

I just tried typing "German folk music" into YouTube, and for comparison "English folk music", "Polish folk music" and "Italian folk music".   Some good stuff (as well as some not so good stuff, of course) on the last three, but the first struck me as a consistently sorry bunch of clips. Which was rather disappointing.