If Nickelback had blown up a few years earlier or a few years later, maybe they aren't hated quite as much. That 2001-2005 range was just abysmal for rock in general, with a bunch of douchey butt-rock bands dominating the landscape. It was pretty obvious at the time that the hard-rock genre had played itself out creatively, and was being chased off the cultural landscape by hip-hop/country/pop-punk. Nickelback became the face of that era, sort of a North American answer to the grip that Coldplay had on Europe during the same timeframe.
I find that younger people tend not to hate (or love) them nearly as much, because they didn't experience that cultural context.