Consider the Great Migration of African-Americans from the Jim Crow south to the industrialising Midwest. 1930-1940 is also the height of the Great Depression a

Consider the Great Migration of African-Americans from the Jim Crow south to the industrialising Midwest. 1930-1940 is also the height of the Great Depression and the Dust Bowl, so people would be migrating from the Plains areas down to California for better opportunities. With regards to the big shift in the 70s, the first thing that comes to mind is widely-available air conditioning making the interior of the southwest more habitable