They just decided X11 did everything wrong and did it differently rather than pick up the pieces (if in spirit of idea, not code) that work and fix parts that don't
This is partly due to the fact that everything you can do with Wayland is defined in protocols that are straightforward to use whereas in X11 you have atoms and messages with arcane name and structures for everything, a lackluster documentation and terrible error handling.
Q: if they had that much experience why they chose to structure wayland in a way that's such PITA to write for ?
A: Because they were reacting to Xorg, so they wrote the exact opposite of that.
And for bonus points, because one of the problems they wanted to solve was "Xorg is hard to maintain", they made sure that the replacement was much much easier to maintain and develop... for them. Not for application devs, not for users, but for the folks making wayland, I have no doubt it's very well streamlined and easy to work on.
Tbh, if that were the case I would expect much faster progress.
There are people opposed to things like a allowing windows to specify their own bounds, and unless all the stakeholders agree to implement such protocols in their respective projects, the ecosystem will remain fragmented. Multiply this against every feature that people want.