AS sells the SEA-ATL-BNA connection on an AS-coded revenue ticket, so MCT is likely not the issue.
DL does use married segment availability, which may affect which AS flights you are able to choose.

If you would consider flying in Y, BNA is often one of the least expensive places to fly from SEA/PDX.

Alternatively, call AS and see if they can ticket the itenerary.

I've often run into married segment logic flying DL to ATL and BNA where there would be no lower fare availability unless one had long connections even though earlier flights had availability as individual flights.