(1) People are comfortable on the streets, they enjoy the freedom, or they are at least accustomed to it, and paying rent/bills seems anathema to them.
(2) People are severely disabled by mental illness or addiction, and simply can't manage a household by themselves. Or they never learned how and they fail dramatically. This may be the 20% contingent who are moving back in with friends/family.
(3) People are naturally generous/irresponsible with belongings and income that they will spontaneously share the windfall with everyone they know. So your shelter for one man will end up being a flophouse for a rotating cast of shady types, and poor housekeeping skills definitely become operative here. Homes like this ultimately run into big trouble, like massive drug raids, fires, prostitution or something. Not fun.
Around here, the brass ring of housing is Section 8, and Section 8 has strict rules about guests and inspections and income paperwork every year. If you're not abiding by the rules then you're out on your ear. I don't know how many people wash out, but my case managers have inundated me with certificates and kudos for being one of their best, longest-term clients.
Housing First type approaches is great, and I'm glad to hear that they've "ended homelessness", even though that seems to sort of be contradicted by the 20% figure that was constantly cited in the article.