My understanding is that that's largely because SF is actually a pretty good (or at least "less awful") place to be homeless in, so homeless people from other areas travel or are sent there in hopes of finding a better life.

It's apparently a thing for social workers and caregivers in places with drastically inadequate social support (i.e. most of the US) to just stick disabled/mentally ill people on a bus to SF; they'll at least have a better chance there.