The physical business of building computer parts, especially chips, involves a lot of hard science and deep magic and requires a kind of priesthood steeped in that magic. SV's original success was built on this -- it got rich because it is where the silicon priests lived. But now things have evolved so the it is the Chinese who know how to build the computers and (sometimes) the Californians program them.
Except writing apps is not really deep magic, and is something that can be done anywhere. So SVs advantages move to ever more nebulous things like networking and mentorship -- which are also human universals that happen everywhere.