I discovered that there are quite a few poly people here in Southern CA (quite a few like to work the ren faires
). The sheer population numbers may be part of the reason. The entire west coast in general seems to be a bit more accepting of alternate relationship styles in general and frankly openly non-monogamous relationships have been around a long time and have become fairly common. Nobody really worries about what the neighbors are going to think, because the neighbors don't really care if you have 2,4 or 8 adults living in your house, this is just normal (high housing prices and large immigrant population).
That's just my opinion.