Well, the simple answer is blame that huge spine of rocks running up and down the height of the country called the Rocky Mountains. Think of the Rockies as both a giant wall and as a weather factory, and the colder weather created there moves from the Rockies and goes only one way - east. It goes all the way east until it gets to the Atlantic. Seattle is west of the Rockies however, and the entire coastal plain there is warmed by the waters of the Pacific ocean and that heat is trapped behind the opposite side of the wall. Seattle and New York get about the same amount of precipitation each year, but Seattle only gets about 4 inches of snow, while New York City averages over 6 times that. Seattle's moisture all falls as rain because it's on the warm side of the wall, while New York eventually gets the colder temperatures and snow that come out of the Rockies and continues to build as it moves across the country. If you want to see a visual depiction of this, a great source is the USDA Plant Hardiness Zone map, and specifically take a look at the west coast.
