As best as we can tell from the experience of various European countries, legalization of sex work has the following benefits: (i) it reduces the exploitation of vulnerable women, (ii) it enables better health care services to be provided, thus reducing the spread of STDs, (iii) it enables better identification of sex trafficking and the exploitation of minors, (iv) it enhances tax collection by moving a black economy activity into the regular economy, thus making pensions & disability payments available to sex workers.

The reason legalization of sex work doesn't happen more often - and is strongly resisted when it does - is because of evolution. Safe comfortable middle-class women unconsciously seek to minimize the risk of family resources being diverted toward other women and so generally oppose legalization in order to protect their own interests. They aren't aware of this, of course; we humans rarely have any insight into why we do the things we do. Women who oppose legalization cite a number of supposedly "moral" reasons for doing so. But it's clear that the desire to limit potential competition is the greatest single factor contributing to why so few nations legalize sex work despite the evident benefits of doing so.

Secondarily the financial interests of professional religionists - those who depend on ordinary people being unhappy so that they cling to myths about invisible magical pixies watching them and offering the lollypop of a happy-ever-after-death life in the clouds - also oppose legalization, again using supposed "morality" to disguise the true nation of their objections.

In the end, as is almost invariably the case, we're just too stupid a species to see the glaringly obvious and do the sensible thing. We prefer infantile fairytales because they offer the illusion of certainty in a complex world most people's brains simply cannot ever encompass.