It's an incredibly high bar to prove sexual abuse, and in the US there are a lot of rape kits that haven't even been tested, which I find really shocking (although the rape conviction rate here is equally dire). I imagine that the difference with GM is that she was involved with several of the women who testified, so the level of evidence is that much greater. Even then the jury deliberated for quite a while.
If the men involved only slept with, say, one of the victims who've come forward, would there be enough evidence to find them guilty beyond reasonable doubt? I'm happy to be proved wrong by someone who's followed the evidence more closely than I have.
I'm thrilled that at least the women have had some kind of vindication. JE is dead and GM has been convicted. I hope, along with the Weinstein conviction, it sends a message to the rich and powerful that they can't just use young women in this way.