Why do people wind up homeless?

Having struggled with homelessness myself over the course of 20 years, and having written extensively on it, and researched aspects of it, I think there's a fairly simple basic answer to this question.  Homelessness is a very complex issue as a whole.  But the main reason people become homeless is not addiction to drugs, alcoholism, or mental illness, those those are factors in most chronic cases.

People become homeless because of the lack of a strong social network of either family or friends that can help them weather a difficult time.

That is the critical difference that leads to homelessness.  The vast majority of addicts in the world have a place to live.  The vast majority of alcoholics in the world have a place to live.  The vast majority of the mentally ill people in the world have a place to live.  The difference is the social network.