Just because they're Christian doesn't mean that they don't have things that they like that often contradicts what their faith is, Alice Cooper is a devout Christian and even that doesn't stop him from putting on material that wouldn't be considered wholesome on stage for example.
If I had to guess though, most metal genres/bands aren't really too keen on organized religion, and while exceptions do exist, they aren't exactly dime a dozen, what with the moral stink they've raised back in the 80's and 90's. Now that the "shock rock" stigma has went away, metalcore is one of the major sub-genres where they can profess their faith while being aggressive enough for others to enjoy, though in the former, they usually don't mention their faith or at least try to be subtle as possible. Of course, metalcore is still a controversial genre regardless of the the Christian influence to metalheads and critics, so there's that.