I am a hub captive who had to shotgun marry DL once AirTran got bought by SWA. I used to exclusively fly AirTran business class because they cared about being on-time, I didn't care about meals & wanted Wi-Fi, while DL in 2008-2011 was a mess.
Remember 10 years ago when DL realized that the majors had to merge and got the first pick of prom dates... NWA.
While UA, CO, US, AA were all flying separately without disturbance, DL and NW struggled to assimilate.
By the time DL and NW started to gel, the other 4 hit turbulence in the early stages of their respective mergers.
So I think DL had the head start of being the first carrier to complete its major merger and has a more cohesive culture which is evident in their operations and profitability. But the other majors are working hard to close that gap.