Well, if you compare bands like Iron Maiden or Pantera to bands like Korn or slipknot, you can see why. Iron Maiden and Pantera pretty much set the stage for mainstream metal 9along with Metallica), and here you have bands with weird growling beatboxing and a band with about 9 people in it.
And I remember hearing that one of the drummers from Slipknot said they were the heaviest band in the world. When I heard that, several bands popped into my head that were heavier than that as I was laughing the hardest I had laughed in months.
I don't see why people hate Nu-metal. I love Slipknot, and I don't mind Disturbed that much. And as for Slipknot being the heaviest band in the world, I completely disagree. I don't know, you just have to listen to something like Souldrainer to see that.