Until recent years, the reason was because colleges had a vested interest in covering up sexual assaults on campus. Victims were routinely advised not to contact law enforcement, and perpetrators faced no punishment.
Now, many colleges routinely contact the police when accusations are made. In many instances, there is not enough evidence to criminally prosecute the accused. The level of proof needed to convict in a criminal case must be beyond a reasonable doubt.
In that same case, however, there might be sufficient proof to show the accused violated the college's rules regarding sexual misconduct. In that case, a college disciplinary hearing would be in order. The DA cannot prosecute people for violating campus behavior rules.