First, women are rarely believed, and don't want to go through what I explained upthread.
Second, many people don't actually like what sexual harassment is constituted as. In the Opening Post to this thread, SSJPhenom admits he doesn't think kissing a colleague is sexual harassment - it is, legally, and his colleague would be legally within their right to say so. But, in situations which may not be seen as 'that bad' or 'not even assault' - are people going to come forward? And, remember, the US voted for an admitted sexual predator to be POTUS, so forgive them if they don't believe they'll be taken seriously.
Third, they just want to forget what happened. There are many accounts of victims saying they felt the reporting and court case felt just as bad as the attack. Most people don't want to relive it over and over.
Fourth, in many cases the attacker is in a position of power. Whether it be teacher, boss, judge, parent, celebrity. Those people can very easily ensure you are not believed, that you don't get to work in your chosen field, that you will be humiliated, that everyone will hate you. They can make victims genuinely believe they are wrong, that what is happening to them is ok.
There are more reasons, as I say, a quick google can explain them far better than I can. The issue with this question, is again, it isn't asked as a genuine question, but as a way to judge the victim, in belief that waiting means lying. When there are hundreds of psychological reasons for waiting if people cared enough to actually want to know, and not just to discredit the victim.