Date rape wasn't prosecutable (in any states that I know of at that time) because the concept of rape meant that the assailant wasn't personally known to the victim. Essentially, if a woman knew anything more about the guy who attacked her than his name, she would be accused of 'leading him on'. And district attorneys would decline to prosecute. This meant that if a woman worked with someone who later raped her, it would not be taken seriously; same thing for a victim and rapist who happened to ride the same bus.