PV touched on quite a few of the contributing factors. When it's such a big problem, it's not simply one reason. On top of all the stuff PV listed, I think some communities have made the regulatory framework so difficult to construct new housing that you also have a supply/demand problem. The supply has been artificially constrained, for various reasons, but the results are still the same... increased housing costs. We all know how freakishly expensive housing is in places like LA and SF.
I want to be compassionate, but with the problem this big, we're going to have to do some tough love to get it under control. How about CA moves them all out to tent cities on the other side of the mountains in the desert. They're alive, but life really sucks, and give them some motivation to get out of that rut.