Or, perhaps, why criminalisation of certain behaviours doesn't. Leave aside which group of prodnoses it is in this case, and have a look at what the Wisdom of Whores  blog has to say about prostitution in Cambodia. Put simply, in the face of the HIV epidemic rather than trying to stamp out prostitution the authorities decided to co-opt the infrastructure itself, to ensure that condoms were used at all times (erm, well, at all times of sexual congress, at least).
HIV infection rates came crashing down, halving in just 5 years. It is estimated that condom promotion had saved 970,000 Cambodians from HIV infection by 2007.
The pressure now though is to close down the sex industry altogether, something that no one has ever managed, thus disrupting the way in which that extant structure has been manipulated to reduce those HIV infection rates. Other than those who think that there's something inherently wrong about the commercialisation of sex, something that's in fact so wrong that it's better to try and fail to wipe it out rather than manage the effects, most people would think of this as rather counter-productive. That the sex workers themselves are demonstrating for the right to remain sex workers might also give some thought.
You don't have to fully sign up to the rather extreme version of liberalism that I do, that ingesting what you wish as you wish or offering your gonads for pay or for play, again, as you wish, is one of your natural human rights, to think that perhaps attempted abolition isn't quite the right way to go about things.
As with drugs and their decriminalisation and needle exchanges, perhaps red light areas, brothels and condoms, are better than 970,000 people being infected with an incurable disease that will kill them young.
From the purely utilitarian point of view, what's best? Reducing the ill effects of what people are going to do anyway or attempting and failing to stop them doing it and ending up with all of those ill effects?