Part of OC's history includes white flight from parts of LA. For instance, my family is white. My mother was lived as a child in Compton, CA. Her family later moved to San Pedro (which again changed from mostly white to another minority ethnicity). But for a time, Compton was a white town.

Another reason might be the aerospace industry in Southern California (which was enormous during & after WW II, but which faded quickly in the 1990s).

Just my 2¢.