Luckily, we can check NZ, since they legalised sex work, and research has since been done to see what the answer was: http://www.gaatw.org/publications/SWorganising/NewZealand-we...
And actually, none of that is true! Actually it turns out that what really enables trafficking and the abuse of sex workers is situations where they can't go to the police, or where brothels have to hide their existence from the authorities entirely. Once you're legal and regulated, it's really easy for the immigration officials to show up and start checking paperwork.
> you'll still fail to have only the entirely above-board, willing sex work that the sex workers themselves would like.
Funny you should say that; again in the linked report it was noted that NZ does have an issue with that, but not from trafficked women, but rather from migrants. Due to anti-trafficking sentiment, the original law legalising prostitution banned people on temporary visas from working in the industry, which means people on temporary visas are the only sex workers in the country who can find themselves subjected to abusive work conditions while being unable to report it to the authorities.
It makes sense, of course; everywhere you ban sex work you create a shadow that victimisation can take place in.
In some places, it's not merely a theory.
Why the discrepancy? Maybe it's geographic as the other commenters suggests. The Netherlands is in the middle of somewhere while New Zealand is in the middle of nowhere. That sounds plausible to me. I think it might also be cultural. The Netherlands is known around the world for their sex industry, New Zealand is not. This makes the Netherlands a more rational destination for profit seeking predators.
It's the same reason why e.g. we ban all new elephant ivory, period-- even ivory that supposedly does not come from poachers killing an endangered, wild elephant. Because if you try to allow "good, fair-trade ivory", the poachers will try their best to get their stuff in there. And when it comes to the balance between costs and benefits, it doesn't take many bad apples to spoil the whole barrel.