It's an amalgamation of several crises. Homelessness, extreme surge in costs of living, poor healthcare (esp. for psychological problems), public toilets, opiate use, ... I guess such things always are. It's pretty much never a single cause.
Remembers me of mid-nineties Germany (where I've grown up) – in Munich (yes, Oktoberfest), Kindergarten teachers did rounds in the morning to clean up the fenced-in Kindergarten playground, remove the syringes the junkies left. In one of the better neighborhoods, too. Whole squares right in the city where hundreds of homeless did drugs with helpless police standing by. I remember being chased away from a playground in a well-off part of Bremen by an approaching mob of violent homeless junkies. Lots of refugees from the Balkan war, often in an absolutely desolate psychological state. That, too, was an amalgamation of lots of crises – reunification, economy doing poorly, a society built by the rules of Cold War, falling apart, war in the Balkans, huge reluctance to change the tiniest thing.
But Germany really turned things around. A lot of targeted action, lots of reforms, lots of small changes to how the social security net functions, a big invest in robust police services, somehow mostly without the Police violence issues the US have. There are some corner cases where the social services still fail to help, other than that, it's become relatively hard to stay homeless for long. In Munich, you really have to know where to look to find any homeless at all (Hofgarten at dusk; one particular river bridge.) Being poor isn't fun, and it's comparatively harder to get out of poverty (though the US seem to be catching up there.) It's easy and cost-neutral (i.E. part of mandatory insurance or covered by social security) to get help with mental health issues – no doubt that contributed. No opiate crisis to speak of (got lucky there).
A lot of money has been (and is being) spent on it though. It'll be interesting to see whether the US will do likewise, or find a way to make it work on a smaller budget, or simply fail to get things to improve.