This is probably because for most crimes, it suffices to establish that (1) the act of the crime occurred and (2) the defendant is the one who carried out this act.

In many types of sex crime cases, the prosecution must also prove that (3) the behavior engaged in by the parties was not consensual.

People do not generally consensually enagege in battery, theft, or murder. People consensually engage in sexual activities all the time. Also, sexual activities often have no third-party witnesses, which makes proof more difficult.