WN's growth on these routes is probably also boosted by its dominance of the business market in California (based on what I've read on these boards). So, while AS may be the go-to airline for business travelers in the Northwest, that only helps for half of the route - going south-north, WN has an equally loyal business base. Perhaps PDX business travelers heading south and back isn't enough to sustain the routes - they need both southbound and northbound customers.