A common thought I see repeated by many commenters is that Seattle is a magnet for the homeless because it is "soft" on the homeless.
Salt Lake City puts the chronically homeless in housing. I don't see many homeless clamoring to go back to Utah.
And, to be honest, having spoken with many homeless, a lot of them are from out of state. But, then, a lot of Seattle's current residents are from out of state, also. They didn't come here for the social services. And neither did most of those who are homeless.
They come here for medical care, from places where they can't get care. They come here for jobs, from areas of high unemployment. They have enough money for a while, but chances are they've given up whatever they had to come here.
Seattle is a magnet for people seeking employment.
Seattle is a magnet for people seeking to create or expand businesses.
Seattle is a magnet for people.
And people sometimes become homeless.