Cost of housing is a big one. Low housing costs and plenty of space available make the homeless problem both less likely and easier to deal with.

Yep, there are plenty of outlying areas in LA that are still not expensive in the same way SF is - LA is just much, much bigger. So you can hang on with significantly less money. Still, it's an interesting question.

Much of it has to be geographical--SF has mostly bearable weather year-round, Boston doesn't.