This question always comes up in cases like this. I can explain it from personal experience, if it helps, having been involved in goings on involving a different "celebrity" a few years ago.
When something like this happens, it is always a shock, even if the person has previously been kind of flirtatious/inappropriate. But the biggest thing is that it's as if they have suddenly given you some kind of power over them, which you never asked for and didn't want. They realise you might tell someone about it, and you have absolutely no idea who you can tell without it getting out and potentially implicating you in a huge scandal. It's as though someone has handed you a dangerous animal to look after: you can't really offload it to anyone else, because they probably won't want it either. And you feel like you're stuck with this enormous burden you're expected to carry on the person's behalf, and over time that becomes too much to bear. Every time you see them being written about in public, every time your colleagues discuss them, you can feel the anger rising and you're powerless to do anything.
It's not so much a case of wanting them to be punished, or to suffer; for me, it was the sense that I wasn't allowed to say it, you know, just actually say out loud what they had done, even if it wasn't that bad in the scheme of things, because it would cause so much trouble. I don't like attention, but I also don't like being expected to keep secrets, because it feels wrong. So that's why I obsessively looked on social media for other people writing about the guy I had the experience with, and when I saw that someone else had said the same, I got in touch with her and it all cascaded from there.
It was horrific, even though I stayed largely anonymous; I told some people I knew, or half-knew, and lots of them contacted me wanting to know more, or to apologise because they were friends with him. I thought he had some serious psychological issues, that's all. But knowing he had assaulted several other women helped me to decide I had to do something.
I sometimes felt like people didn't really believe me and it made a lot of those former acquaintanceships quite awkward, too awkward to continue in some cases. So I regret it in a sense. But I don't feel that there was much else I could do. It was simply getting too heavy to bear, especially as he would frequently write about how wrong it was for people to get away with sexual assault. The hypocrisy was too galling to watch and the anger I felt was harming my own mental state. So that's just my perspective, as to why wait. Because you don't feel brave enough, and it could cost you a great deal if you say anything.