It has been this way for a long time. One team that my younger son played on for a while was invited to fill a gap in the Surf Cup tournament that had occurred because one of the invited teams, one of the Celtic boys teams of that era, had won State Cup and the Western Regional Tournament and had a schedule conflict with the National Tournament. My son's team played State Cup champions from three different eastern states, won their group easily, and then were knocked off by Celtic's other boys team who went on to win Surf Cup.Don't know what you're talking about when you said "Socal allows any team to join" maybe you meant SOCAL league?
What I meant is that Southern California can field enough talent that parents and players don't need to travel to AZ or NV for games. (Not that I hate going to AZ or NV) it's just not nessasary when the big clubs in Socal (location not league) are the best in the nation. It's a very clear signal that there's much more opportunities for more teams. They might not be leading leagues in Socal but they'll be better than 80% of the equivalent clubs nationwide.