It probably correlates more to age than anything. SF is a more popular place to live if you are young, single, and outgoing (i.e. more to do there). Mountain View (and areas like PA, Menlo Park, etc) are much more appealing to people with families.
And introverts. I've never understood the complaints about the South Bay being socially dead. I've got a very busy social life here with a number of friends within walking distance, it's just my idea of socializing involves going out to dinner or over to someone's house for Starcraft or board games or a movie instead of doing a pub crawl of half a dozen bars or having a raucous house party.
