Probably because in my experience most men despise rapists (shocking I know) and the rapists are probably very well aware of that and the potential legal consequences they can face whereas a rape victim can (at least in a circle of friends that they trust, I know there can be serious issues when it comes to going to the authorities) usually expect a certain level of empathy and support.

Which of these groups do you think is more likely to open up about the topic to close friends? Couple that with the problem that a lot of rape victims out of shame, guilt, fear or the problem with proving even legitimate cases go to the authorities too late (or in some cases never) it can very well be that a rapist can easily fly under the radar for a long time unless you spend an unrealistic amount of time together.

Add to the way man and women talk about sex among friends in general. Even when they talk about sex I rarely see men give more than a superficial report (‚I got laid‘) and rarely ask in depth follow up question whereas my female friends usually immediately ask for all the details of how exactly things went down.