I saw an article which mentioned Delta would beging using the A220 once a day between ATL and SEA. The article speculated this was to do test runs for future transcontinental service using this aircraft.
I suppose this would be the right sized aircraft for long thin routes like CHS-LAX, JAX-LAX,LAX-ORF,LAX-PVD etc.
Maybe Delta wants customer feedback before using this aircraft for transcontinental service to medium size cities.
The efficiency of this aircraft with a few additional FC seats may be very profitable for Delta.