Aside from zoning laws, there's the lack of a unified federal intervention. This prevents any one area from addressing the local homeless issue because any area that takes steps to address it will consequently absorb more homeless individuals from other places in the country. For example, if a city in California develops a program to house any homeless individuals, then homeless individuals from other cities and states will be more likely to go to said city to get housed. Even worse, there are states that would actually pay for their transportation. What would happen is that either the city would have to solve a much larger homeless problem as new homeless move into town, or the initial wave of homeless people will be house while the new arrivals and homeless will stay homeless, leaving a continued homeless problem.