I think WN had LAX-SEA non-stops at one point? I could be thinking of a Seattle trip a couple years ago, but I think I compared AS vs WN for a trip to SEA this past November (and being surprised that WN was just as expensive as AS, UA, etc - LAX-SEA always seems higher than it should be). Then again, I could be imagining things and they never flew direct.

Don't forget the DL RJ service they would have to compete with soon. Definitely another reason to avoid SEA-LAX (since so many people will want to spend 2.5 hours in an RJ)! As justageek said, I think it is just because WN has better things to do than compete with AS.