What drove
Silicon Valley from the 1950's through the 1980's was Lockheed Space and Missiles Division, based in Sunnyvale next to the then-Moffett Naval Air Station. The development of the Polaris, Poseiden and Trident SLBM's were done there, and so did most of the reconnaissance satellites for the CIA, too.
As such, there was a huge need for engineers, and Silicon Valley initially grew up supporting this Lockheed division. Essentially, the computer industry took over by the middle to late 1980's as Lockheed would down operations in Sunnyvale.