Not only do they tend to select larger cities, but also warmer cities. You mostly hear about the homeless population in places like San Francisco, Oahu, Austin, etc. If you're homeless, you might as well go to a place where you're less likely to freeze to death. Although there are some homeless in places likes Pittsburgh, Philadelphia, etc.

Not to mention some cities will give them money if they take a bus to another city. Just shuffle the people around so it's no longer that current city's problem.