Probably because it is very exploitative of women, spreads disease and is rejected by the very same moral people who use the services of prostitutes.
Legalization of prostitution helps very few prostitutes. Most of them are left in the same abysmal circumstances as they are in where prostitution isn't legalized.
Prostitutes in a lot of European countries will disagree. They have mandatory medical care, legal assistance and in some countries they even wanted to unionize
And they pay taxes and have pensions as they age![]()
And there is still illegal prostitution, still kidnapping for sex slavery, whores still spread disease and pimps still provide specialties.
The net effect is zero. It doesn't really help anyone and wouldn't even cut down on arrests.