Because it can appeal to anybody, which is not what metal is about. It's supposed to be a rare artform for those who have the insight to understand the intelligence, artistic notion, and emotional side which makes metal metal...
I have to admit it, many American Nu Metal bands stole the label of metal because they did the following things:
A. Make loud music.
B. Semi growling (Slipknot, KoRn, etc)
C. Have a moronic fanbase.
Bands SUCH AS KORN took the harsh vocal styles without crediting bands such as Death. In the eyes of the majority the most famous band IS the band who has made it and metal can't reach that level because people have generalized the sound and mentality behind it as music for "angry, loud, fat, drunken idiots".
That is my say on the whole "mainstream metal" argument. Mainly it's about the modern bands. Trivium, A7F, Slipknot, etc... who haven't truly done anything innovative and are only famous because they created a sound that those who DIDN'T EVEN LIKE METAL previously can get into. It wasn't for that crowd unless they can hack the real thing.