Afaics this is because there are no other forms of evidence available in sexual assault cases where what's being contested is consent. Two people have had sex, but testify differently over whether one of them said "yes". How do you figure out which one is telling the truth? I know it's very distressing for the victim, but if we keep "innocent until proven guilty" while making circumstantial evidence inadmissible then I don't see how anyone could ever be successfully prosecuted for this kind of assault