It's mostly the cost of housing. The rate of homelessness goes up and down in direct relation to the cost of housing as a fraction of median income. What you also see is places with high rates of drug addiction and mental illness, but low rates of homelessness like West Virginia. Because housing costs are low there. Even a drug addict can scrape together enough to stay dry.

What you are seeing is that when the cost of housing goes up it's the people on the margins of society who are pushed onto the streets first and have the hardest time getting back into stable housing. That doesn't mean the cost of housing wasn't the driving force.