Main reason is because she flopped, and if you flop you're automatically disliked. If she came back with a multiplatinum album they'd be kissing her ass

Also a lot of it is bandwagoning. A couple people make fun of her, so everyone else does because they want to fit in and it's so pathetic and lame. Some Britney stans are still stuck in the early 2000's where Brit and X were rivals so they pick on her just because of that even though everyone else has moved on
