Victims of sexual assault are filing reports now more than ever. (page 2) People who are arrested for rape have between a 50-54% chance of being convicted. (page 10.) The reason so many people are reluctant to step forward is that apparently, 50% of people who meet the legal criteria of having been sexually assaulted don't consider their experiences to have been rape. (pages 2-3).
There's no catch-all explanation for why some people don't report this stuff. Some people are embarrassed, some people don't feel they're victims, and some people fear being humiliated. It's waaaaaay complex.