Wayland on its own may be ready but you can’t build a whole desktop with just Wayland. The rest of the stack needs time to catch up.^(*) And no, not everybody is willing to use KDE and restrict themselves to whatever combination of elements happens to work right now.
^(*) Because the bright people who did this decided they needed to throw the baby out with the bathwater on X. They couldn’t possibly find a way to ditch only the obsolete parts and fix the problems and maintain compatibility as much as possible. No, everything had to be rewritten from scratch.
So here we are 15 years later, with another 5 or so to go until the whole Linux desktop ecosystem will be thoroughly redone.