The main reason why women would be less likely to report sexual assaults (when compared to other crimes) is that they would be likely to fear the way they might be treated by the police and, if it comes to that, at a trial.

Female victims of sexual assault often feel as if the attack was in some way their fault.  More than that, they fear that they will be treated as if it had been their fault.  This can be a very intimidating thing to face up to.  Women will be afraid that they will be grilled harshly by police and prosecutors who want to be sure of the strength of the case before bringing it to court.  If the case goes to a trial, they will naturally fear that the defense will question them closely about their sexual past or their conduct with the defendant.  In essence, they fear that they themselves will be put on trial.  This can often feel like a second victimization. 

Because they fear the ways in which they will be treated in the legal process, women may be hesitant to report sexual assaults.