This is no surprise. DL has more fortress hubs than AA or UA and is aggressive about maintaining dominance. B6 tried to fly to ATL but got squashed by DL. WN had to buy FL to get a foothold there.

AA and UA have largely kept LCCs out of ORD, DFW, MIA and IAH for their part. LCC service goes to other nearby airports for the most part. DL hubs like ATL, DTW and MSP do not have any alternates, so they can put the squeeze on the entire market.