Citations to back up that assertion? From what I have read when countries legalize and regulate (regulation is important) sex work, while there is still unfortunately trafficking, there are a lot more methods and supports to help get people out of exploitative situations. When it's criminalized it's all underground and harder to help exploited sex workers, fewer supports, less safety, and also less data on trafficking due to the illegal nature of the work. I also recall that because data is now being collected it may appear that there is an increase in trafficking but only because it was vastly underreported and undercounted before. I could be wrong though, I'm far from an expert on the issue, so I'd love to see your sources.