I would be willing to bet that some of the problem that DL as well as AA, UA, et al's IT is that fundamentally airline systems are based on tech created in the 60's-80's. Deltamatic, Sabre, Amadeus, etc. Sure they've all slapped some nice GUI's on top. But they're all basically DOS.
Part of the problem is it is about the most difficult thing an airline can do to switch GDS's, much less switch GDS's to something totally new. Then complicating things further is that even if say Delta designed a totally new GDS from the ground up...it's still going to have to "talk" to Sabre, Amadeus, etc because that's what travel agencies use..and I don't mean Barb's discount travel in the strip mall, I mean AMEX, CWT that are worth billions of corporate and high end leisure travel.
And you have to make the switch while still operating the airline because other than 9/11 there is no time that there are no Delta planes in the air or Delta tickets being sold.( I think they turned off ticket sales for the "Boxing Day" mistake fare, but that cost them millions in tickets and lost sales)
This is really just a very long winded way of saying, that I feel bad for airline IT people...they may be some of the best in the world (no clue but I"m sure they all employ some very bright people) but the true beating heart of virtually every airline GDS is ancient and limited.