Because X11 is a huge dumpster fire that no one wants to maintain. How would you feel about maintaining a 30 year old legacy piece of software which has changed hands 5-6 times since its inception? Wayland is truly a better design and everyone is in favor of leaving that codebase behind.
At that point, why not just deprecate and replace the two(?) things that are problematic with X11: the keyboard logging protocol, and lack of access control on screenshots?
Surely, that is easier than reimplementing the entire graphics stack, all window managers, and all applications.