Part of it is the routine stuff around activist-abolitionist DAs, lack of housing, NIMBY-ism, weather, decriminalization of drugs, etc. The review Shellenberger's comprehensive book on west-coast homelessness is nice level-headed take [3]

But one major cause is the rise of potent p2p meth & fentanyl in the populace. [1][2]

So not only are the homeless un-shelter-able (shelters require residents to be clean), but the nature of drugs lead to irreversible mental health issues and irreversible (ie. chronic) homelessness.

[1] https://dynomight.net/homeless-crisis/#:~:text=To%20get%20mo...

[2] https://dynomight.net/p2p-meth/

[3] https://astralcodexten.substack.com/p/book-review-san-fransi...