1. Variety. You can climb glaciers, alpine ice, some big walls, alpine rock, volcanoes, world class trad cragging (Index), and winter desert cragging, plus winter mountaineering and bachcountry skiing. Neither Colorado or California can offer all that.
2. Scenery. Go do some routes in the Pickets then go hike a couple Colorado or California 14ers. There is NO comparison. I love the change in scenery avaialable from the North Cascades to the Olypics to the Pasayten.