lol!
It seems that in Germany, prostitutes have to pay taxes, which could be a huge factor in it:
Prostitutes have to pay income taxes and even have to charge VAT for their services, to be paid to the tax office. In practice, prostitution is a cash business and taxes are not always paid, though enforcement has recently been strengthened. The Länder North Rhine-Westfalia, Baden Württemberg and Berlin have initiated a system where prostitutes have to pay their taxes in advance, a set amount per day, to be collected and paid to tax authorities by the brothel owners. North Rhine-Westfalia charges 25 euros per day per prostitute, while Berlin charges 30 euros. In May 2007 authorities were considering plans for a uniform country-wide system charging 25 euros per day.[10]
I would be supportive of it being kept illegal if otherwise you had to pay taxes to the government.