The Central Valley, particularly the San Joaquin Valley, receive high numbers of first-generation Mexican immigrants with low levels of education. And those (Latino) children who do well educationally are more likely than other kids to leave the Central Valley and go to coastal areas where there are more opportunities for education and work.
There are a lot of reasons why California is no longer as strong a location of choice for international migrants as it once was. Housing is one. In California and in San Diego, wages for low-skilled jobs aren't very different than those in Arizona and Nevada, but the cost of housing is much higher here.