Because she wasn't just posting your average run of the mill sexy content, she was doing full on softcore porn after awhile. Dick ratings for guys, selling worn panties, butthole and nipple "slips" showing herself seemingly having anal sex with her boyfriend in the shower, etc. Not shocking a publicly traded PG company with tons of sponsors and stockholders wouldn't want one of their stars doing that kind of stuff. If Disney signed an actor to play a Disney Princess and she started doing the stuff Mandy was you can guarantee they'd tell her to stop and fire her if she didn't.