It's mostly coz nobody really wants to improve X11. I don't think there is many wayland features that would be impossible to implement in X11 it's just nobody wants to dig into crusty codebase to do it.
And sadly wayland decided to just not learn any lessons from X11 and it shows.
What do you mean nobody wants to improve X11? There were developers with dozens of open merge requests with numerous improvements to X11 that were being actively ignored/held back by IBM/Red Hat because they wanted Wayland, their corporate project, to succeed instead.
Not having enough maintainers, and some design issues that can't be solved are both reasons why X was left largely unmaintained.
There were a lot of MRs with valuable changes however Red Hat wanted certain features to be exclusive to Wayland to make the alternative more appealing to people so they actively blocked these MRs from progressing.
> someone could have forked the project and be very happy with all the changes, right?
That's precisely what happened, one of the biggest contributors and maintainers got bullied by Red Hat from the project for trying to make X11 work and decided to create X11Libre (https://github.com/X11Libre/xserver) which is now getting all these fancy features that previously were not possible to get into X11 due to Red Hat actively sabotaging the project in their attempt to turn Linux into their own corporate equivalent of Windows/macOS.