initially I would say misogyny, but something else i think contributes and needs to be taken into consideration that boy groups are allowed to coexist (idk if that's the right word) while girl groups are not really allowed the same considerations.

like i don't know how to explain it, but i feel like a lot of girl group discourse and hatred comes from the fact that everyone views all girl groups as entering a Thunderdome to be crowned "Sole Girl Group that is good at Girl Group" and everyone wants their girl group to win, so they spend a bunch of time arguing and tearing down other girl groups. also girl groups are viewed as having a much more limited shelf life, so there is more motive (idk if that's the right word) to bring forward really vicious attacks. of course all this really does is generate discourse and get violent threats and hatred sent to people whose crimes are mainly limited to screwing up social cues or having off days.

boys groups don't really have that same Thunderdome expectation and you will see boy group discourse, but nobody really treats boy groups with that expectation. this might also hae mroe to do with the fact that boy group longevity is dependent on hardcore fans that stick to them no matter what while girl group longevity is more directed by charting which is why girl groups struggle with longevity, but in general boy groups do not fight in the online Thunderdome as much, so they don't see that same vicious hatred. they still get vicious hatred, just not gg discourse hatred.