I can tell that it's painful when your city's hub is smaller than other hubs, and it sure hurts when people like me trash-talk your city. I get that DTW'ers get offended when I talk about your city being in the rust belt and not actually recovering from the recession.
The problem is that the hurt can't negate the truth. Your downtown is recovering, but DTW metro area is still in the dumps. The broader catchment area, whether defined strictly or broadly, contains cities in dismal situations and with no prospects. Look no further than Flint, 60 miles away, to see that grim picture. A lot of DTW's connecting traffic (at least traffic brought in by RJ's) comes from other parts of the Rust Belt.
Thus, it is only normal to expect that Detroit will have less service than SF, Dallas, or Atlanta. Those cities are in a better economic position, as are their broader areas. If Delta has to choose between two otherwise equal (or near equal) hubs, they will naturally choose the one that can get them better traffic from its area, and the natural connections. ATL and the deep south are more affluent than DTW and the midwest, so DL would rather add routes to Atlanta.
OK, now you're surely wondering why ORD is so successful. The answer is that ORD sucks in traffic with connections from everything between the Rockies and the Mississippi heading east, while offering a possible connection for East Coast travellers heading west. That's how midwest hubs normally work, except Delta is special: MSP and DTW split this niche. While ORD transfers passengers from every direction, MSP/DTW have to share that traffic between them. That results in stunted service for each, but perhaps a superior product for Delta as a whole.
So why is DTW "getting shafted by ATL?"
1. Detroit is still a poor city
2. DTW and MSP are "splitting a hub"
TLV, BRU, ZRH, CDG, FRA, EWR, JFK, DEN, SFO, AUS, RNO, SEA, YYC, YYZ, IAH, ATL, IAD, DCA, ORD, SJC, SNA