Because sometimes those charged with keeping a safe environment act with indifference.
Here's a current example: http://www.news-leader.com/article/20110816/NEWS12/110816008/Lawsuit-filed-against-Republic-School-District-over-rape-claim
and http://thinkprogress.org/justice/2011/08/17/297888/missouri-school-sued-for-allegedly-making-special-ed-student-write-apology-letter-to-her-rapist/ (same story, 2 articles).
In this story, a special education middle-school girl is penalized for reporting harassment and assault by a classmate. She's even made, by school authorities, to write an apology and hand-deliver it to her rapist. Then she's expelled from school, for allegedly making a false report.
When she's allowed to return to the school the next year, she is again harassed and assaulted by the same boy. School authorities again do not believe her, but this time her mother takes her to a Child Advocacy Center for an exam. Seems the exam showed she had been raped; semen found on her is said to match that of the boy. Boy is arrested, charged, and plead guilty.
Family files lawsuit against School District. School District replies that the girl failed to use reasonable means to protect herself, the suit against them is frivolous, and hence the School District defendants are entitled to be awarded attorneys' fees.
So, it's the girl's fault. Again.
And that's still another reason that sexual assault is rarely reported.