I'm going to float it out there that it's pretty hard to get someone to report sexual assault in the first place. Add the beloved celebrity part to it and I think it's easy to see why people may have waited. Beverly Johnson mentions she didn't want to take down a successful African-American man. I think this can be compared, as much as I hate to bring it up, the priest abuse cases.
Will we ever know the truth? Hard to say. It just seems like a lot of people coming out of the woodwork.
If anything, I think Bill Cosby was more beloved now (or immediately prior to the accusations) than he was early in his career.