I've actually thought about this more than I should. To me, Nickelback has a lot in common with Taylor Swift. Their songs use a formula that is almost universally liked. It's like an AI analyzed all musical hits in history and spit out "Rules for a hit song" that they both follow.
For example, one pattern I've noticed is the choruses often use a pattern of [Looooong note. Loooooong note.][Short short short short short note]. Their biggest hits all have this. People seem to love it.
Anyways, my point is it's enjoyable but formulaic music. The difference is that Taylor Swift's image embraces this more. She's pure pop, dance, love-song. Nickelback, however, tried to advertise themselves as a serious rock band. I think people sense the dissonance and have been angered.