That is because studies by the Swedish government have shown that the criminalization of clients did not in fact make a considerable difference in the amount of people doing sex work. What it did, though, because of the harassment and targeting of sex workers on the street and the difficulty in accessing clientele, was to move a lot of them indoors, with a lot of them starting to work in massage parlours and more out of sight.
On July 7th, 2014. See this statement in context.