Tech debt accumulation from the 1950's onwards to now basically. The GDS are integrated with everything from check-in, rebooking, ops center, etc. Lots of companies looking to change this, and some airlines have been able to modernize. But it's a fraught process that can lead to mass cancellations during the upgrade period as stuff breaks.
To whit - Southwest has moved (is moving still?) to Amadeus and there has been a fair amount of reporting on the issues as they happen.