I think it's primarily because AS
has such a strong hold on the northwest. UA
is a shadow of it's former self on these routes, Delta pulled out, and Virgin America is starting up. Throw in token services by JetBlue and it just gets to be a bit much for WN
to fight for.
I do think we'll eventually see them do so, but with the plethora of one-stops via RNO
, etc, you really are only a short stopover away from being nonstop anyhow. While I'm atypical, I actually enjoy the little stopover excursion - we've been to RNO
most recently (same plane - just stop and go) and I liked it.