I believe it's due to metalheads not usually being fans of rap music, or maybe because those two genres aren't that similar to each other, their roots aren't the same so the combination sounds a bit off. Like if you mix EDM with country: "Goin' up yoo-o-o-o-o-nder" - imagine how hilarious that would sound like. Or it could be the commercialization of it that really vexes some people, same could go for emo/metalcore and the like. Metal is a bit of an underground music genre and people generally don't like it when big companies try to temper with it.

But I'm not completely sure. I'll agree that some nu metal sounds cool, especially when the music has elements of groove metal (I like Sepultura's Roots, for example).