Simple supply and demand are the reasons for the higher prices. Orange County has some 30+ cities and LA County has 91+ cities. San Diego County has something like 17 cities. I don’t have exact population figures, but do the math. There are simply more buyers looking to find homes in those areas.
You also have to factor in all those people who now live in the Inland Empire because of the job opportunities on the other end of the 91 and 10 freeways. They are likely adding to the demand of housing in LA and OC.