I'd imagine the main reason is that SF is walkable - not only for the homeless themselves, but also for the targets of their begging activity. The weather is liveable, I suppose, but LA's is better - if that's all it was, they'd be in LA, but LA's much more car-based.

Anyway, whatever the reason, I think it is actually a "situation". Many cities have homeless problems, but SF's is amazingly in your face. Most SF people I know would simply not walk through Tenderloin at any time, even in broad daylight. It's in the middle of the freaking city! That is not normal.