I feel like there's a few reasons for this.
1) Fans want to have bragging rights. They want to continuously shove their faves achievements in other fandoms faces. I've witnessed this a lot with Armys, Blinks, Onces, etc. It just gives people this mentality that their faves are "better" than everyone else's.
2) Bandwagon fans will want to join the hype train and then hop off of it when a group starts declining in sales, charts, etc. Or they might just stan people for their looks....
3) Some people just don't care about music quality anymore. They care more about their faves getting all of these music show wins, how high their faves are charting, etc. (This one kind of ties in with the first point I made)
Honestly, I'm now getting to where I don't care about music show wins. I mean of course I still like it when my faves get an award, especially if it's their first win, but I'm not going to be totally upset if they don't chart well or win as many awards as possible. I care about how good the music is. I could care less about sales and charting numbers. That's why I'm getting to where I'm wanting to unstan a few groups. It's solely because I'm liking their music less and less despite how well they may be charting....
But anyways, hopefully this all makes sense.