There are probably a hundred ways to rationalize why. Inertia, lock-in, obstinate grey breards refusing to retire and holding infrastructure hostage until they die... , if-it-ain't-broke-don't-fix-it-even-when-it-is-broke, "we can't afford to stop the world for your cargo-cult modernization rewrite, sonny...", too big to fail, on and on...
It's a pretty weird situation, but by my estimate, it's probably going to require totally new airlines rolling their own systems from scratch, in parallel isolation, next to the huge legacy carriers as they continue to operate, in order to transition, modernize and unseat the current technology. That seems to be the way things work in industries this big.
Fast food, banking, telecommunications. All of them have examples of modern upstarts that are now world-wide staples, simply because they started from scratch with more modern efficient systems, as they continue to co-exist alongside dinosaurs that still haven't gone away.
[0] https://en.wikipedia.org/wiki/Global_distribution_system