It all depends on the crime.
Take the case of a victim of sexual assault. Sometimes, it is a matter of support available. Some people are truly afraid of talking about what happened. In some cases, the victims want to talk to the police but do not want to go to court. That's an important decision to make. Sometimes, there's a lack of trust. People may not even know where to go or who to talk to. I gave the example of a victim who might confide in a teacher or a friend.
That said, we hope that people report these crimes to the police, since it's important to do so and these are very serious crimes.