it's cause there are so many Asians in Northern California. Most of the prostitution is because of all those massage shops and brothels that they run out here. If you remove all of them, you would probably see the same amount of prostitutes as anywhere else.
Another thing is... In the Bay Area, there are literally undreds of thousands of engineers with lots of money, no game skills, and no time to really date around. There is more men than women, and Asians and Indians have trouble on the dating scene. You simply have a lot of lonely guys out there with money. It's a recipe for hooking success really.