A lot of it probably has to do with the fact that many people think (a) Chad Kroeger is an annoying vocalist (b) Nickelback plays generic pop/rock, (c) Nickelback's lyrics are extremely stupid, uncreative and un-original and (d) Nickelback is all over the place, headlining shows in big venues. Meanwhile hard-working, more talented bands like Alter Bridge, Submersed, 12 Stones, Dark New Day etc. go largely unnoticed. Yeah, Nickelback used to write decent songs....back in 1998. The latest stuff, in the opinion of many, has just been, well, blah.
As for why I personally hate Nickelback.....three reasons: annoying vocals, horribly overplayed generic songs and disgusting lyrics. Look at a song like "Figured You Out" or "Animals". The lyrical content of their latest stuff just disgusts me. I guess I tend to expect better lyrics out of so-called "rock" bands. It seems like Nickelback has lyrics that are more in line with that of a 50 Cent album.