Very good question? I wonder why the west and California has the most productions?
Probably a lot of reasons. Sammy mentioned a good one above, but more and more I've been hearing of girls getting their start in some little midwest town as a stripper and then moving out to LA specifically to become pornstars, so I'm not sure its quite as true as it used to be. Even if the the stars themselves aren't getting sucked in out of desperation, other aspects of the industry may be. In addition to actors/actresses, LA also has a glut of videographers, video editors, aspiring independent filmmakers, and so on. And they have cheap equipment and sets to rent at a moments notice. So the general filmmaking culture of the region makes it easy to do porn there. And, of course, you've got all the other regional perks of working in SoCal... perfect weather, gorgeous scenery, cheap plastic surgeons, a thriving high-life drug culture...