Maybe because no one bothered to improve it for over a decade? ;)

The Wayland developers have limited manpower. Obviously it would take a long them for them to get Wayland to reach a critical mass when there wasn't support from the community.

Sort of feels like a self-fulfilling prophecy.

The real problem was that Wayland was extremely bare-bones initially and needed many extensions to approach the usefulness of X11 for desktop use - it basically just supported dispatching input and blitting rectangles. And initially, getting extensions standardized was like pulling teeth. These days, "obvious" extensions get in in a matter of months (about one DE release cycle), not to mention that most already exist by now.