Window managers for X11 need to be rewritten as Wayland compositors (every single one of them) if somebody wants to use them as a basis for Wayland desktop environment.
There are several Wayland compositors. Mutter (from Gnome3) is only one of them.
Weston is reference Wayland compositor.
KDE5 has KWin compositor. Developers from KDE Plasma project recently have invested a lot of time and work to refactor KWin for Wayland. You can read some more about KDE Five Plasma's KWin work here:
https://blog.martin-graesslin.com/blog/tag/wayland/
There is also a Sway Wayland compositor inspired by i3 X11 window manager, but nowadays integrated desktop environments (Gnome3, KDE5) are best bet for using Wayland, though.
It is sad and I hope this is going to change, but it is what it is, at least for now.