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.
I've only been in the TL for three years or so, but I've spent the majority of that time wondering how regular working-class families with kids can afford to live in the area. I can't imagine how street-level pushers or prostitutes make rent. Maybe they rent places out in Hunters Point or Outer Sunset?
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.