Because most of them probably felt that bringing that accusation to light wasn't worth the trouble it would cause them personally, but when he flat out lied about it in the middle of a Presidential Debate, it was a bridge too far. That, and the fact that once other women came out about their experiences, it meant that they wouldn't be alone in the media spotlight, as a figure to be distrusted. Others had shared their experience, which allows for some solidarity.
When validating the first two stories, at least, the New York Times made sure to ask others who knew the women, and said that the women had indeed told them about these assaults all those decades ago. Unlikely that all 20+ people for each woman was lying about it. Certainly a lot less likely than Trump lying about it, which actively serves his interests.
Besides, why is this even a controversy? Are we going to believe the women who say they were groped by a man, or the man who said he gropes women? This isn't "he said she said." This is "they're both saying the same damn thing."