good is the enemy of excellent. X11 works for most users (almost all the users?) well. You can see that with the adoptions of other standards like the C++ standards and IPV6 which can feel like forever.
Another thing I think one of the X11 maintainers mentioned iirc is that they have been fairly gentle with deprecation. some commercial company could have deprecated X11 and left you with a wayland session that is inferior in some ways.