This is fairly new, and because the Valley has become such a boring soulless place, whereas San Francisco is now and always has been a very attractive city. Attractive enough that techies pay a big rent premium to live up there. And, there is an increasing amount of programmer type employment in SF.
Ha
Yes, they call it the "San Francisco cloud corridor" (that's where I live), home of Twitter, Zynga, Yelp, and many others.