Because jesusfreakos still maintain a Vulcan deathgrip on the shorthairs of politicians. Prostitution is illegal for the same reason that buttsucking censorship is not only legal, but de rigueur....Victorian prudes (i.e. religio-fanatics) know what is best for everyone else, and if they cannot force their beliefs on you by way of an outright theocracy, will do so by means of The Nanny State.