A lot of it is not readily buildable due to terrain, proximity to water supply and "protected" environments.
Most of southern California is a desert. Los Angeles could not exist if water was not brought in to it.
California is no more uninhabitable because of it's terrain then any other parts of this nation. We have technology and know how to build anywhere.
As for "protected" environment, these area became protected by law and can be unprotected by law.
The only thing that restrains California from being a great state again is the strangle hold the socialist (with support of unions and government workers) have on it.
The Democrats have created the mess and it will not be fixed until they are out of power.