I haven't been to Seattle, but I've been to other west-coast cities and I think their city planners just do a better job with managing the intersection lights.
New York City's lights are pretty good, but they're on a timer and some are pretty long. Because of how the lights can chain together, there can be long gaps of no cars even though the light says "don't walk." Since there's tons of people at the corners and no cars, people jaywalk.
When I've been to California, the lights are on much quicker cycles and are more pedestrian friendly. Whenever I've drive out there, I found myself thinking "Ah crap, I just missed the light." And then it changes, and I'm on my way. It probably ends up keeping the general speed down as people try not to speed through intersections, and pedestrians know that if they just wait at the light for a few seconds, they'll be able to cross.
I lived in Baltimore for a while and there were some notorious lights that would last for minutes across an otherwise dead intersection. People jaywalked like crazy because waiting at the intersection was a great way to waste 3-4 minutes.
Sure enough, a quick search shows that Seattle has been focusing for about 10 years now on signal optimization: http://www.seattle.gov/transportation/signaloptimization.htm