I had this same frame of mind just a few months ago. "Why the fuck is Slipknot top billed over Slayer and Motorhead?" Then I saw them and was like "holy shit...ok" They are headlining Download Festival 2013 along with Iron Maiden and Rammstein. And they are twice as popular as those bands are in the US. I believe that the only metal band that is bigger in the US than Slipknot is Metallica.
I know. And it will never happen because that would be the most expensive LN which act ever. But yeah, Slipknots fanbase is mostly composed of like the metal version of the ICP scumbag fans. It is highly unlikely that they would play any of the big four festivals, because their fans would rather pay just to see them than 5x that much to see an assortment of shit that they don't care about. metal festival or none.