Why do people hate Nickelback so much? What makes them different from other popular bands like Linkin Park, Green Day, etc.? Is it just because they're Canadian or what?

Those opposed to the band are simply louder and more vocal than the true fans. A matter of opinions.

There will always be that one guy or gal. They may have friends also, so opinions spread, growing in strength , spreading yet further until the humble opinion of one becomes a near fact to many.

Even I have cracked a few jokes about the band but I did so not intending any ill will or malice toward the band. Going with the flow at the moment I believe as NickleBack was the topic of the moment.

A Glucose Monitor for Someone Without Diabetes: Optimal or Overkill? - WSJ

Canadians are generally likeable people , so NickleBack are most likely ok guys.

BoB's answer to Is Stan from Eminem’s Guilty Conscience the same Stan from Stan?

NickleBack may appeal to many, but the majority rules as the saying goes and the overwhelming majority of more vocal proponents voice the opinion the music is bland, generic and commercial, not necessarily in any particular order.

While you sleep, these bugs throw a party on your face - East Idaho News

NickelBack.

Anyway, why are they so despised in comparison to Green Day, Linkin Park, etc ?? To use an analogy, it is like comparing the smoking/stoner crowd in high school, the athletes and finally the unpopular souls.

I doubt many hate them for being Canadian. I was going to say no one, but never say never , so to speak.

Why has my ex moved on so fast after years of being together with me?