Upvote:9
Alaska and Delta have both flown PDX-DTW nonstops in the recent past (5 years).
Alaska had a redeye in 2017, but as reported at FlyerTalk and confirmed in their archived January 2019 timetable, this was changed to a seasonal route and then discontinued altogether.
Delta inherited the DTW hub (and PDX-DTW) with its 2008 merger with Northwest Airlines. According to their archived November 2019 timetable, they were operating one flight per day during the off season.
My armchair analyst's suspicion is that there is not enough demand for this route to justify more service. Alaska would want to route connecting passengers through its larger hub at SEA, and as for Delta, they have been focused on building up JFK and LAX, although the route is likely to return after the COVID-19 shock fades.
As for low-cost carriers, Southwest and Spirit serve both airports, but neither seems to have been interested in the route.