As you mentioned, there is a seasonality component to Alaska travel, and UA's hubs are not ideally located to provide year round ANC service. I can see why DL/AS provide round service on SEA-ANC because of the historical ties between the two cities. I thought CO used to have a year-round IAH-ANC flight that was reduced to seasonal service because of the fall in oil prices.

Also, I think the ORD-ANC is year round but is less than daily during the winter.