People who care about which music is cool are mostly young, urban and liberal or progressive. They hate Nickelback because they're Canadian rednecks from Alberta. Everything fits that picture: the meaningless band name, the bland middle-of-the-road rock, the simple fake-romantic lyrics, the sexist videos, Chad Kroeger's douchy looks and self-important behavior, and the unsophisticated style of the fans that they do have. BTW, I think many people who answer this question on the Web produce a rationalization about musical issues because they're unaware of their prejudices.
Funny thing I learned just now: the only Nickelback song I had saved on my PC and MP3 player was quite a challenging piece of music, I thought, since so much of it was sung in canon. Now it turns out this wasn't one song but two: someone simply played their hits 'How You Remind Me' and 'Someday' at the same time - and the harmonics are exactly the same.
I love a good mashup so thanks for "This is How You Remind Me of Someday" lol! They're not the first band to turn formulaic though. From what you’re saying I take it they have the right combination of hatable traits for a lot of people to latch onto. Those traits have little to no effect on me so I wouldn’t ‘get it’. But who were people hating on musically before Nickelback?