Slipknot escape it because they weren't ripping off Pearl Jam but forgetting that "credibility" thing. Joey Jordison being an insanely talented drummer certainly helps, as well, and an argument can be made that they're the ones really responsible for the semi-mainstream acceptance of extreme metal from the early 00's. Mick Thompson is also an underrated and underutilized guitarist to boot, and I think Corey Taylor gets shit on unjustly when it comes to his versatility as a vocalist. Basically, they were a gimmick band that struck at just the right time and (purposefully) spaced out their releases enough to make them always seem fresh in the mainstream rock world.
That they also sounded very different from their "nu-metal" contemporaries (with maybe only System of a Down coming close to the level of intensity) doesn't hurt their case. I think 90% of their stuff is shit, but I'll admit that a lot of their songs would be great metal tunes if they didn't rely on sing-song choruses that completely change the feel. To put it further, they're basically a child's introduction to
real metal, similar to how Motley Crue was in the 80's.
Rendclaw said:
Hell another band who got this treatment (and still got it until very recently when it suddenly became cool to like them) was Rush, who got fucked with for the better part of 40 years until 2010 more or less, when they put out their documentary. This kind of thing is nothing new. Every so often a band comes along that for one reason or another, people just love to hate for no good reason.
The big difference was that Rush was popular for being amazing on a technical level. Nickelback are popular for being lowest common denominator tripe. They are the Journey of their generation: completely manufactured, soulless, and disposable bubblegum pop rock of the era that went from virtually beloved by all to the butt (rock) of every joke, and then nostalgia will kick in and people will listen to them because it's "fun."