Possibly because some would argue that the prostitutes are victims of life. OK a huge generalistion but we do hear tales of women forced to work the trade for a number of reasons. Illegal entry into the country and the need to fly under the radar can produce sex slaves too scared to report the abuse they recieve from their pimps so as not to alert the authorities to their existence?

Legal brothels would presumably ensure the girls are here legally. So the illegals would still be offering cut price thrills on the streets. Not sure of the solution to that.

Other girls claim to hate the business but need to do it for a fix.

I would personally rather see the money that would have to be ploughed into legalising it go to setting up rehab projects.