What really hurt Edmonton--and a lot of other Canadian airports--was the pilot shortage at United, even before COVID.
United used to really serve Edmonton well, with nonstops to Denver, Chicago O'Hare, San Francisco and Houston--all big Star Alliance hubs. The SFO service was dropped but subsequently revived by Air Canada. But most frustratingly the Chicago service never came back.
United had also made a major commitment to other Canadian cities, starting service to Denver and Chicago from Regina and Saskatoon, to Chicago from Thunder Bay and to San Francisco from Victoria. All of those services were subsequently cut, with United blaming a lack of pilots.
It would be so great to see a carrier step into the void and revive some of these routes. Frontier tried offering service to Calgary a number of years back but was pushed out because of heavy competition on the Denver route from (who else?) Perhaps the carrier could be attracted to serve Edmonton-Chicago Midway (Frontier is establishing a focus city at MDW). Frontier is looking to expand and has been pursuing Spirit...perhaps we can get a Chicago nonstop again and Regina and Saskatoon can see a return of service to DEN.