Perhaps the reason that San Francisco is so relentlessly progressive in its politics is that the residents can all see first hand the divide between the rich and the poor?
I've always thought of it as the other way around. A less "progressive" city would have cracked down on all the bums, vagrants and low-level crime long ago. One Giuliani-style mayor and the Tenderloin would look like... well, Times Square.
(Admittedly it would probably just kick the problem to the other side of the Bay.)