Actually if you look up the re-offense rates by crimes, I believe sex offenders have the lowest re-offense rates, with only murderers being above them.
If I were to guess, this is probably due to the length of the prison sentences given to these offenders. A thief can get slapped with a small sentence, come back out, and steal again. But if you do a 10 or 20 year rape charge, and are then released as a registered sex offender, where basically nobody wants to associate with you. There is both not an incentive to reoffend, nor is there really a means to reoffend.