I think it stems from every one of their songs sounding like an imitation of “smells like teen spirit”. Except that Kurt cobain was- or at least is seen as- authentic, raw and vulnerable, whereas nickelback comes across as a pretentious imitation loaded with macho posturing. Kurt would have hated them, which is why many others do.

Besides that, their music is really boring- in the sense of being extremely predictable. Lots of other music you hear on the radio is boring too, of course. But it’s a double whammy.