The Wayland developers have a security model [-1] that is hostile to "power-users" (those who like to use the Unix (or other OS) programming environment to its full potential) and the visually impaired (eg., blind). See [0] [1] [2] to see what features I am talking about.
It is possible to implement some of those features on a per-compositor basis, but the result of that will be graphical API fragmentation, as programs that interact with GUIs will need to have separate code for each compositor. And the work is not done even for Gnome (more precisely Gnome's Wayland compositor and the Gnome applications that use it) yet.
On the other hand one could say, eg. "Why not make a compositor accessibility protocol on top of Wayland?". End result of that, it is easy to guess, would be something worse than X Windows (because of even more layers of abstraction, and possibly even more incompatible standards/APIs/protocols), which the Wayland people were supposedly trying to escape from.
Edit: Another thing that makes Wayland (at least without an extension ...) unsuitable to replace X Windows is forced compositing. This means unavoidable double buffering and thus worse video performance (especially noticeable for interactive stuff like video games).
[-1] I prefer calling it security theater, because it does not bring any real security improvement in practice.
[0] https://news.ycombinator.com/item?id=20308011