Seattle has steeper hills than San Francisco. And the times that it snows and actually sticks is probably once every 10 years, on average. And when it actually does do that, it sticks around for about 3-5 days before it melts. It's not like the midwest where you'll be driving in snow for months. So there's no real budget for dealing with snow (plows, salt, sand), and no one has any practice driving on snow. And no one owns winter tires because you'd use them once every 10 years, for about a week, and then you'd have to get new ones, because tires age. Combine that with our hills, and, you get disaster. There's a photo of a bus that slid on a hill and almost went through a barrier and into I-5. The front end of it is hanging over I-5.
Plus, we get a very moist snow, due to our proximity to a warm Pacific current. That stuff easily turns to ice once it's compacted.