Because like any scandal it's less about what the person did and if they're a liability. When was hired Gunn most didn't know about his past tweets so he wasn't a liability and he made them tons of money now the vast majority knows and a company that most of what they produce it aimed at kids don't need one for their biggest brands connected to a guy who the public now knows made pedophilia jokes.
Rosanne wasn't fired because of racism she's been making Xenophobic and Racist comments for a few years now since 9/11 basically. She was fired because the public lashed onto one recent racist comment and now it was going to cost ABC and Disney money. Same with Gunn it's always about Money Hollywood will protect you no matter your sins if they think you can make them money and throw you to the wolves if they think you will cost them money end of story.