The incentives of living in SF versus, say, Mountain View or Palo Alto seem to be nightlife. And most founders I know aren't really going out very much. They're too busy with their startups. It would seem Mountain View is more ideal for that.
Maybe MV is better for super-early stage startups with just two or three cofounders, and as soon as hiring occurs then SF becomes more appropriate.
It seems to me that SF has more distractions. I of course could be totally wrong, and I'm curious why, if so, the trend continues.