For me, it's twofold.
I live in the province that they originally came from. Canada has a lot of rules when it comes to what gets played on the radio/television (this was before YouTube or online media streaming was even close to being remotely A Thing.) Very very generally, it's about every third song has to be "Canadian content" (generally, by a Canadian artist/performer.)
Nickelback was coming right at the end of what was a really cool Canadian musical renaissance.
A ton of Canadian bands were emerging and were very much deserving of radio play based on their own merits, and a lot of support was being built up for homegrown music.
Nickelback included. I actually enjoyed their first 'mainstream' album, The State. I remember seeing them being the first opener for Everclear way back when...some other shitty band from Los Angeles was the second act.
Anyway, Silver Side Up got released in 2001 or whenever it was, and 'How You Remind Me' became inescapable. You'd hear it at the doctor's office, while on hold for your insurance company, driving to work, at work, in your casket as it was being lowered into the ground. It was everywhere. This was partly because a lot of the Canadian acts at the time were self-imploding or going on "hiatus", so the CanCon regulators milked the fuck out of Nickelback.
It was a horrible feedback loop -- Nickelback was popular, so CanCon kept playing them, and the reason CanCon kept playing them was because Nickelback was popular.
Then Chad Kroeger decided he wanted to be a label owner, so he started up his own thing and started signing bands that sounded almost indistinguishable. One of those bands, Default, became almost as huge as Nickelback in Canada and one of their songs, "Wasting My Time", became a hardcore playlist staple for CanCon regulators.
Then you have another soundalike, Theory of a Deadman, who was also signed to Kroeger's label and it starts getting a little vexing.
But that in and of itself wasn't a bad thing. Music fads come and go and you'll always get people trying to cash in on shit. The grunge/alternative fad of the 90s had a TON of bands that sounded very similar to each other. It happens.
But unlike grunge, Nickelback/Default/et al. were everywhere. They managed to tap into the top 40 chart, the rock charts, the "adult contemporary" (which is such a weird fucking category because it can include everything from Frank Sinatra to Cher to, yes, Nickelback.) You couldn't get away from it.
Imagine if you went to the movie theatre, wanting to see a comedy, and you ended up getting Michael Bay.
Then you go to Blockbuster and you want to rent a nice classic film, but there's only Nickelback. That's what it felt like.
Nowadays, you don't really have this being jammed down your throat anymore because we have so many media options, but as a Canadian who was around at Nickelback's height, it was fucking horrible.
I also don't really care for him after he said he's fine with being the McDonald's of music. Mostly because I thought that Nickelback was an interesting band way back when, but they just opted to go the easy (and creatively bankrupt) route.
Sorry for the essay, haha.