As someone who helped design this:

Whatevs.

Alaska used to be 70% of the gates at Seatac. They're now 50%. Delta has more gates only because they ate Northwest. Prior to Northwest being sent to swim with the fishies, Delta had zero gates at Seatac. And the only reason anyone flies Delta now is it's the only way to get to the Midwest.

If you're flying into or out of Seattle, the two airlines you pick from are Virgin America and Alaska. Virgin because they have wifi and generally better terminals, Alaska because their mileage plan slays Virgin. If I check in online on an Alaska flight to Los Angeles that I bought with my Alaska Airlines Visa, I get 2500 miles... and that doesn't include the 2-miles-per-dollar I already got for buying the tickets. If I fly Virgin I get 925. Meanwhile, 20k miles gets me to Hawaii non-stop and $115 gets me a companion seat.

Virgin is nicer to fly, tho.

Alaska's death grip on SeaTac is such that they control the runway fees. Southwest got cranky a decade back and threatened to start flying out of Paine Field if they weren't lowered. SeaTac told them to pound sand. Then Alaska said "no, those Southwest guys kinda have a point" and the fees came down. Delta? That's the shitty company that killed Northwest that everyone will fly anything else first (except for American). Fortunately with American your only choices are JFK or Miami.

Delta? Delta's that airline you have to fly if you need to go to Madison, Minneapolis, Cleveland or Chicago. And may God have mercy on your soul.