There's many reasons why people might take so long to come forward. I'm sure most have been mentioned.
The fear of not getting a conviction in court would likely be a major factor as many cases are "he said, she said" as the phrase goes (the genders vary obviously).
I reckon soft sentencing also plays a role. To be the victim of a sexual assault is very traumatising so it'd be quite confronting to see your experience trivialised by the spectre of such a weak sentence.
And one factor which probably hasn't been mentioned would be the desire to protect loved ones/parents from the pain of knowing. So maybe some victims wait until their parents pass away before they come forward.
I know from my own experience I'd hate for my parents to know what happened to me as a boy. They'd be crushed and I don't want them to live with the guilt. I can carry the burden on my own. Some kids grow up being Souths fans so I always tell myself that it could have been a lot worse!
And by no means am I saying that coming forward is selfish. That'd be ridiculous. But eveyone deals with it differently and crucially the way you perceive your experiences, and your options of resolving them, evolve over time.
Ideally you'd come forward straight away but I suspect, like me, many victims aren't even capable of processing wtf happened at the time.
And if you don't come forward straight away, the powerful force of 'time' takes over and it becomes very easy for 1-2 years to become 1-2 decades.
It's certainly a very complex situation; which many people fail to understand when they're all too quick to suspiciously blurt out, "why didn't they come forward earlier?!"