You have all the universities (UCB, Stanford, Caltech, UCLA) as well as a critical mass of tech companies that allows interchange of staff and the creation of a new company overnight. Getting a new job is as easy as going out for lunch. This applies to the East coast as well. Neither area is lumbered with a large unemployed population claiming benefits.
Having so many corporations means that a startup can remain in stealth mode and keep under the radar of politicians and quangocrats. I've known companies to implode because the local government office instructed that they were instructed "not to promote anyone any further but instead to have a fresh talent initiative".
Weather isn't that much of a factor. Even places close to the Arctic circle can have a strong tech base providing the quality of life is high.