Without claiming to be a victim or to speak for victims, I think I have some insight into why victims of sexual misconduct find it difficult to come forward at any time, let alone immediately. I don't claim this is definitive; I do think it a useful starting point in evaluating claims, both of guilt and innocence.
I spent the better part of a decade as a commanding officer in units ranging from about 300 total personnel on up. Several incidents during that period — years before law school — shaped my approach to these matters. Twice, I was the CO of a victim reporting a sexual assault; once, of the alleged perpetrator. Two of the reports were within two days of the alleged event; one was substantially later. (I also investigated a few incidents at the direction of higher authority.) None of the incidents, however, involved significant physical evidence that either supported or undermined the initial report. Trying to get truthful, accurate statements out of anyone involved, and out of any witness to the incident or its context, was incredibly difficult, and no doubt was quite difficult for the individuals being interviewed.
Those being interviewed know full well that the interviewer is going to be ripping off whatever bandages have been applied, whatever stitches were put in, and probe the wounds. Without anesthesia. With probably unclean fingers, in a very official setting and certainly not one's own space... especially when those fingers connect to bars or oak leaves on the shoulder instead of stripes on the sleeves, because power dynamics sure as hell enter into this. If you really don't think there are power dynamics involved — even for civilians — you've never raised a kid… and probably never been one.
It's incredibly hard to just listen. Not interrupt to ask for clarification on what matters to the interviewer's perspective. Not judge. Not bring in external perceptions of credibility, especially when those perceptions are at best second-hand and relate to things like "job performance" instead of what matters. Not improperly prompt a subject toward what the subject thinks the interviewer wants to hear. Not reject "I don't know" and "I don't remember" as valid responses. And, simultaneously, not ignore all of the problems with human perception and memory from even those with the best of intentions and no impairments.
And that's not as hard as volunteering to have those wounds probed, especially when expecting to not be believed, and when they haven't healed.