I think the 2 main reasons are:
1. LA and the West Coast are where the fish from S. America and Asia land first in the US. All the breeders in those countries can ship to S. California cheaper than they can to the rest of the country. That makes fishkeeping cheaper here than in places like Kentucky or New York.
2. Asians. There's 2 types of Asians -- the kind that like to eat fish and the kind that like to raise fish. (I'm asian so I can say that). I think most of the people I've met through MFK are Asian and just about all the fish store owners are Asian (esp. Thai). Plus, most of the hardcore fishkeepers are Asian. Other races are more into dogs or they would spend the money on saltwater setups. I guess it's a cultural thing that we've always done while other groups have their own thing.