Because one of the biggest issues with sex work, human trafficking, gets worse with legalization.
Yes, because legalizing sex work is just criminalizing sex work with extra steps. It’s very easy to see an (alleged) “rise in sex trafficking” when the legalization shuffle allows politicians to all of a sudden decide what is “allowed” sex work and what is “sex trafficking.”
This is why shitty studies like the one you linked is so thoroughly non-credible - it was performed without the input of the people who actually know what they are talking about - ie, sex workers themselves.