As a filthy filthy hobo I can share my own reasons: nice weather, free college, interesting people, not too hard to find gig work, and surrounded by natural beauty. It's one of the best places to be homeless. Weather and ascetics are a big factor IMO. Even filthy hobos can enjoy the lawns of the Presido, the jagged hills of the city, the almost ominous, creeping fog, pregnant with mystery... and you know, the crack scene is totally popping.