I think what's missing in the analysis is Delta's labor costs in relation to groups. While I can't speak to airlines, I used to work in hotels in various positions like Revenue Management. For every group that was organized, there would be 2 or 3 that were not. They'd miss deadlines, ignore rules like no duplicate names to hold multiple rooms, or submit material in the wrong format. Additionally, taking payments from groups tended to be far more challenging.
Dealing with issues related to group travel takes times. In select cases, a lot of time. Businesses have to pay labor costs for that time.
Chances are, some finance person compared the cost of offering group travel (administered by non-travel agents) with the profit generated by those groups. I would be surprised to learn that Delta wasn't making much (if any) money on most groups.