Population density + cost of living (primarily influenced by housing costs).
If commodities were free, you still need housing to survive.
If housing is free, you still need commodities to survive.
This is outside of substance abuse issues, which is not often a cause of homelessness. How many people do we all know who are functioning/undiagnosed addicts? It's the cost of living that crushes the lowest income earners. The mentally ill have been left to fend for themselves, for decades.