Certain areas are inherently easier to be homeless in, regardless of local programs.

Try sleeping outside for a year in Phoenix or in Minneapolis. Try getting resources in a sprawling suburb without access to a car. It seems clear that SF, with its dense, walkable layout, access to public transit, and year round moderate climate, would be vastly preferable to most areas of the US.

People and resources are concentrated in certain areas

https://xkcd.com/1138/

Because certain areas of the country have better services for homeless people, because certain areas are livable/hospitable year-round, because some places bus homeless people to other areas? Probably mostly the first 2 reasons but the third doesn't help.