[abalone]

Geography's a factor. It's not just SF, it's Oakland too, and there's a ton of commuting between them on BART on a daily basis due to the cost of living difference. They're separated by water so with BART (an underwater subway) you avoid bridge traffic. Also parking in SF and real estate in general is at a premium due to it being on the head of a peninsula.

SF alone might rank lower. Muni is certainly not "popular" with the population.. it's much less reliable than BART. My terribly anecdotal impression is a lot of people bike or take cabs within SF if at all possible.