We've talked about it a few times before -
here,
here and
here. Until now there hasn't really been
enough forum activity to justify it. South California seems to fit with the southwest forum. Most of the US regions are a total mystery to me, to be honest, but the US does seem to have rather more than its fair share - there are only three for the whole of Europe, and one each for Africa, Asia, Australasia, and South/Central America and Mexico.
As Paul said, the number of forums we have at the moment are on the verge of overwhelming our system, but the new software is nearly ready and I understand it's going to be much easier to create new forums, and plenty of them, when it's up and running.