I'm gonna out on a limb here and speculate, but I think it might have to do with the fact that the 2 remaining announced yet unbookable routes (SEA-LHR / SEA-FCO) are gated by pending Boeing 787 aircraft deliveries since those routes will use the new brand-agnostic livery and unlike SEA-NRT / SEA-ICN / SEA-KEF not fly on any existing aircraft. Thus the exact timeline could very well depend on Boeing's delivery timeline.