My guess is that if you were flying out of a regional airport you were flying non mainline and might have been dealing with contract workers, if find both delta and american don’t always have the best of information as it relates to their non mainline partners, gate agents are going by what they see in the system, some will go the extra mile and do research but they are just doing what I could do for the most part, how fast you get rebooked can be dependent on how easy it is to rebook you, as not everyone on a flight has the same full itinerary,

United has just as many issues as aa in my considerable experience, delta seems to give agents the most authority which can be helpful

in the end it is all tough going when things go wrong,