All the big Southern California aerospace spending was big in WWII and for a decade or two after (Lockheed, Douglas, Hughes North American Aviation, &c.), but for various reasons in the 60s congress started moving those contracts elsewhere, especially throughout the South (e.g. a lot of NASA stuff went to Florida). Perhaps the kind of tech that was being done in Silicon Valley had a more plausible transition to non-defense uses, or perhaps defense-related contracts just continued a bit longer there than in SoCal?