Stigma is the why. Same reason people find it difficult to talk about mental health issues. Attitudes about women's history, where a man gets laid a lot that is cool, when a women does something similar they are whores. When you go against a foe that seem undefeatable and what might be said that negatively affects what people believe. Just a couple of guesses
I can see a kid hiding a traumatic experience but as you grow into adulthood I cannot see burying something like the Trump thing and then by mere circumstance she just happens to come forward while it would hurt him the most
The older I get the less I believe in coincidences, maybe its just me
