X is old and very hard to maintain. A lot of rules about how displays work have changed drastically since X became a thing. X went along with most of those changes, which meant the introduction of more and more hacks to keep it running.

Over time X became worse and worse to work on and people realized that it's easier to write something new from scratch instead of trying to fix the decade-old technical debt in X.

That new thing was Wayland and over time most if not all people that where interested in working on desktop compositing pivoted away from X.

Wayland (as it is always the case with new software of that size) didn't hit the ground running. It had various issues at the beginning and also follows a different desig philosophy than X.

Despite a lot of issues being fixed some people are still very vocal about not wanting to use wayland for one reason or another. While some of those reasons are valid, most come from ignorance or laziness to adapt.