Washington has become very polarized in recent years. Most voters were already decided months before the election based on partisian lines. The race only seemed like a tossup because she won such a close election in 2000, and the gubernatorial election in 2004 was extremely close.
My state is split strongly by East and West. If you looked ten years ago, Locke and Murray were winning counties in Eastern Washington easily, but the last time a Democrat won a county in Eastern Washington for a major race was 2000 when Locke was reelected. If Murray couldn't win any counties East of the Cascades, then Cantwell probably won't.
Cantwell probably could win Clark, but it would be difficult, but I would find it more likely than any counties in Eastern Washington (although there is always room for a surprise).
Agreed Washington has become extremely east-west polar, though the possibility of winning Spokane county is possible, I wouldnt be too suprised if Cantwell won Clark this year (Kerry only lost it by about 1-2% and Murray won it in the same year)