Here's an example of one reason (of many) that some women decline to file reports when they're raped…
Bill Cosby has reportedly hired private investigators in order to dig up dirt on his alleged rape victims and discredit them. The American comedian, who has been fighting an onslaught of accusations that he sexually assaulted more than two dozen women over many years, had paid six-figure fees to investigators to find information, Fox News reported. Sources confirmed that Cosby, through his Hollywood attorney Martin Singer, was implementing a scorched-earth strategy in which anything negative in his accusers' pasts was fair game.
If they could turn up proof that one of Cosby's accusers had a history of filing bogus rape charges, that might be relevant. But it's probably going to be more like, "So-and-So was arrested for shoplifting a bracelet when she was thirteen so you can't believe her when she claims Cosby raped her eleven years later."
Everything about the above story may be untrue. He may not be spending six figures to dig up dirt and I can't imagine where any reporter would have gotten the thing about "fair game." But just the fact that this might be done to any woman who was raped will scare many of them off. And then years later, if they finally get up the courage to report it, some will say, "If it was true, why did she wait so long?"