People with chemical addictions will not go to shelters because shelters don't allow you to be drunk or high.
So shelter beds go unused, and people shoot up and drink in parks, on sidewalks, and on busses.
Shelter bed usage in Seattle runs at 90% as of last year, with low reported use rate before that being an artifact of bad reporting rather than reflecting actual conditions [1].
[1]: https://www.seattletimes.com/seattle-news/homeless/qa-with-p...
There are several thousand shelter beds that go empty each night in LA County, include nearly a thousand in Skid Row, because the shelters have sobriety requirements and most of the homeless would rather be high than housed, especially in the summer.
In the winter, it's usually easier to convince the homeless to knock off the contraband and use the shelters, especially after the first dozen or so weather-related exposure deaths.