It's the music and performance for me.
When I got into kpop (2014-15), girl groups were still very into the cutesy and I'm not a fan of that extra high-pitched style. All music for me starts with the vocals, altho I appreciate a good bop. I'll often pick out the voices in a group and recognize those before I know the members' names.
EXO for me were about the music and the voices, then performance and dance and yes, they're fine af (I'm not trying to act like that doesn't matter but there are other people I think are fine af but I don't care for the music or vocals, so that alone is not enough to carry me through) and I like their personalities. Didn't think I'd have another ult, then NCT and the various sub-units snuck up on me in a similar way even as I was paying less attention at that time to kpop. Although the sound of kpop gg started to shift slightly, in general Twice and Bp and groups like OMG just weren't for me. (I have always liked RV BUT I dislike the red stuff so much I don't think I could say I ult or stan them if I don't like half their disco LOL) I also really really love choreo - it's one of the coolest things about kpop - and the gg choreo just wasn't it. (Yes, I understand that some dancers were limited by the expectations of gg and I get that but..0
But this is why I ult some bgs and like some ggs.
I think we started to see a shift in gg styles post-BP with more emphasis on girl crush, which got me listening to more gg and checking them out after years of avoiding them. This was about bigger vocals vs high pitched cute ones and stronger dance performance and more complex choreo.
Now I even ult a gg - Purple Kiss! And Billlie and XG certainly come at me hard. Both groups are heavily grounded in the vocal power (big r&b vocals) and dance performance I love.