Delta's revenue management determines what fares and routings it wants to offer on what routes. Period. Just because the combination doesn't show up on Delta.com doesn't mean it isn't available. If revenue management filed the fare, they want it to be bookable. They believe it represents the best balance of supply and demand that will generate the most profit.

Besides the common sense economic answer, I also have a practical response. If you hit refresh, sometimes ATL shows back up. Sometimes it doesn't. It's clearly a bug, and NOT the intended function.