If someone is "down on their luck" it's pretty easy to get government assistance and qualify for a whole bunch of welfare and job programs. So usually a regular person will be homeless only temporarily if something like that happens. Chronic homelessness is for people who are homeless long term and is almost always accompanied by drug use or mental illness which prevent the person from taking advantage of the various available benefits. For example some programs don't allow drug users to use their resources, which rules out a good chunk of chronic homelessness right out the gate. Or the addiction or illness make it so the person can't hold down a job, so they continually have to rely on temporary shelter and aid.