Well, because there's more than just one justification for starters.
Typically the justification has never really been about limiting sex trafficking so much as limiting the spread of disease and avoiding the moral decay of society by allowing people to act in salacious and sinful ways, as well as by flooding it with single mothers and bastard children.
That eliminates the first example above, but not the other two. And if your present concern is abuse and exploitation of women, then the first example also stands. Seriously, you could prevent most rape if you simply covered women up and locked women away to keep them safe from male predators and didn't allow them outside the house without a male chaperone to protect them like Muslim societies push. Or you could lock chastity belts on all men as Toni suggested, etc.
What you need to justify a ban on prostitution is good data with high numbers (more than what Toni has presented) that it will prevent rape and sex trafficking and other exploitation. Have you considered that sex traffickers may WANT prostitution to remain illegal? It drives prices up and allows blackmail and other pressures against both prostitutes and johns. How much of the legal sex industry is rape and sex trafficking of unwilling women? And does shutting it down help or hurt those women? That's not a question I'm prepared to assume the answer to without very convincing data especially if the answer may the the polar opposite of that assumption. This is about restricting freedom and we should be very careful with that.
