There is a plethora of reasons why women feel unable to report when sexual assault and rape immediately after they have occurred. Victim blaming, shame, fear of not being believed, the fear of losing their integrity, losing their job. etc, fear of wasting their own time, the time of authorities. The list goes on. My wife was the victim of sexual assault, and it took weeks of speaking to her to get her to even consider speaking to the police, and in the end she didn't. The person who did that to her was the partner of a work colleague as well as someone she considered a friend, whereby reporting it would have opened a can of worms, complicating a working relationship and a friendship, and in the end she didn't want to create that difficult situation.
Besides, the actual conviction rate of sexual assault and rape is incredibly low. It's understandable why many may think it is a waste of time and resources at the time it happens.