Because full legalization would allow for stronger regulation and protections that would disincentivize patronizing of trafficked prostitution. We can plainly

Because full legalization would allow for stronger regulation and protections that would disincentivize patronizing of trafficked prostitution. We can plainly see the advantage with drugs, it's the same thing. At the outset, decriminalization of marijuana increases trafficking from the underworld in the short-run. Legalization eventually leads to a regulated market that circumvents illegal channels, and decimates their market share. This is beginning to be the case with marijuana. There's no reason to believe it would be any different for sex work. Germany unveiled new regulations just a few years ago, stay tuned for new statistics this decade.