I'm from Seattle, and while I'm not an expert, I personally think it's because of lack of housing. Portland, Seattle, and LA don't really have public housing in a large capacity. SF has some really old public housing blocks, but they've also been lagging.

Seattle in particular tore down one of it's only public housing projects, Yesler Terrace, and planned to redevelop it. The redevelopment however is similar to what we do here, in that it's market rate housing with some low income properties. This does not make up for the low income housing they displaced. That paired with skyrocketing rents, it's a trickle down situation where people eventually get pushed out on to the street.

Once that happens, there's few opportunities to find a shelter. Seattle also lacks good city run shelters, so a lot of the burden is taken by Catholic shelters in Pioneer Square. Those buildings simply don't have the space, and are extremely restrictive. All of this compounds to a massive population that are basically working for minimum wage, but live on the streets or in tent cities.

Vancouver Canada's Downtown East side is essentially an exacerbated version of this, as the rent spike was quicker there and the salaries lower. It's always been a bad neighborhood, but has just kind of become a dumping ground that the city would like to ignore.

Every city on the west coast kind of follows this model. It all comes down to them not having the services that NYC and many east coast cities started adopting during the Jacob Riis era. Every city in the US is basically capable of having the problems west coast cities have, but haven't had to contend with them because there isn't enough demand to move to those cities.