SEA and DEN have in common a very long average stage length.

DEN is far from most places apart from SLC and has limited express short haul.

Out of all the hubs I'd bet that SEA has the least short haul as well. It's really set up for SEA O&D, and feeding passengers into transpacific from medium and large markets that don't have TPAC of their own. SLC and LAX are both more connection oriented. So, SEA also has long stage length. Which dilutes RASM.

Longer stage length = more fuel and crew hours, so it certainly has an impact on margin, but I wonder if SEA is really that different or if there is just less high margin short haul to prop it up. If that's the case, adding short haul might not help if the demand isn't there (or is filled mostly by AS), and strategically speaking it's not what they built SEA for.