California has had trouble kick starting desalination. It is an energy intensive process, and the California electricity grid (especially in water poor SoCal) is still heavily dependent on fossil fuels (even some coal plants in other states feed into the LA metro). Furthermore, desalination intakes can negatively impact marine life, unless subsurface wells are used. Those are more expensive and companies pursuing desalination are not eager to foot the bill for them. Perhaps most importantly, desalination (even without subsurface wells) is more expensive than current sources of water in Southern California. I've not even mentioned the challenges in achieving safe exhaust of brine.