How are you going to tax it? Most porn is now produced by overseas companies through the Internet. The brick-and-mortar porn businesses in the San Fernando Valley are a dying breed.
And will continue to be produced overseas. The problem is recruitment. In a healthy, prosperous country, pretty girls have better options than porn. In a not quite so prosperous country (like Eastern Europe in the 90's or Brazil now) there are tons of pretty girls who will do absolutely anything for a few thousand.