Because they give a bad name to Metal...Because of them(and how commercial they are...) everyone who don't know shit about Metal think its only noises and screaming when there is nothing more melodic and more intelligent than some Metal bands... + Their lyrics are really dumb and the music is overall uninspired and noisy! I prefer music with intelligent lyrics than dumb lyrics like: I'll kill you, fuck the world, satan ownz, etc... They are mostly a band for kids/teens, I should say... They are posers who pretend to be satanic because kids think satan is cool...
and I see this band very oftenly in some 'best Metal band ever' poll, but its so far from being one of the best Metal band! What about Iron Maiden, Judas Priest, Dream Theater, Helloween, Queensryche, Blind Guardian, King Diamond, Accept, Loudness, Black Sabbath, Deep Purple and so much more ... ?? Now these are some of the best ever! Surely not a commercial band like Slipknot...
I also have the feeling that those who voted for Slipknot didnt heard much Metal bands...
Well, except that I don't really hate Slipknot!:1orglaugh