"If everybody knows, why isn't he in prison?"

Well, because "everyone" can't just go to the cops with "I heard this guy is a rapist"; we'd have to present evidence of specific incidents. And the only people who could do that--the women he assaulted--have opted to leave town and cut off contact rather than try to go through the court system. I don't blame them, given how little they have to gain and how much they have to lose if they go to the police with a story that begins "so I picked this guy up at a sex party, and..."

It doesn't even matter whether they should have done this (although in my opinion, they should do whatever the hell they want because someone does not incur obligations by being raped); it's what they did do, it's what a lot of survivors do, and hopefully it's helped them to be safe and heal, but we can't put the guy in prison based on "we all know what really happened here."