I don't think it's income inequality. It's legality-of-prostitution inequality. That is, the traffickers decided to move their operations from countries where

I don't think it's income inequality. It's legality-of-prostitution inequality. That is, the traffickers decided to move their operations from countries where prostitution was illegal to countries where it's legal, because, all else being equal, it's easier to operate in the latter. The link doesn't claim that the total amount of trafficking increased, and the quoted results don't favor the "net increase" hypothesis over the "movement" hypothesis.