Weinstein being a scumbag wasn't a secret. There have been articles and talk about it for decades. The only reason he is getting pushed out now and all of the complicit/enabling celebrities are willing to talk about it is because he's had a string of failures and was becoming rapidly irrelevant in the industry.
There is some discussion now that his brother/partner Bob gave info to the
NY Times as part of a play to have him ousted from the company.
Also, it's a different world than it was even just a few years ago. Not so long ago, victims who wanted to take action had to do so through police where as we know the bias against victims and women remains strong, and through the legal system where it's very difficult to go after someone with immense resources who can assemble the best team and finance the long process of investigation, discovery, trial, appeal etc. And with much of this happening behind closed doors, the victims might often have felt alone, like it was their word against a powerful person and a biassed system.
Now we have social media, and thus a direct and immediate connection with the public as well as other victims who find courage in the public support and knowledge that they are not alone.
As for others who may have known someone who was personally victimized and should have said something, I'm sure many of them gave some warnings privately though probably vaguely as to not out the victim, though I'm sure many others ignored it or pretended it didn't happen or even enabled it.
Angelina Jolie said that after it happened to her, she refused to work with him again, and warned others as well. Other celebrities have said that they knew or suspected, and now feel ashamed they didn't say anything. In some ways I sympathize though - it's not like Weinstein is/was the only one; it's been part of the culture of this industry (and many others) since the beginning, and thus by choosing a Hollywood career you find a way to deal with it, sometimes by ignoring it because it's so rampant you figure all women (and many men) are in the same boat anyway.
We know how society treats women who come out with sexual assault stories.
Sad but so very much true. However, I'm hopeful that the tides are turning as more and more well-known, respected and influential people come forward. Taylor Swift's recent court victory, and importantly her honest and unapologetic stance throughout, has likely had a positive effect and may have even given some of the Weinstein's victims the confidence to go public now.