Different standard of proof is the main thing. In many of these cases, absent a rape kit or any other evidence, it's a he-said-she-said, and that doesn't work well with "beyond a reasonable doubt". Criminal cases also can't be settled out with money, only lesser charges, which might still make him a felon and/or put him on a registry. Don't flame me, but there is also less vengeance and retribution for the victim in a criminal case. Civil cases are a different standard (preponderance of the evidence; think "50/50"), and can be settled out with money, directly to the victim.