While an X11 window manager just manages windows, a Wayland "window manager" has to also do everything an X11 server does (including X11). It doesn't just manage windows.

There are display-server-as-a-library libraries you can link with, so that you can focus on writing the window management code you actually wanted to write, but you're still responsible for everything you link with, so it's far from trivial.

Also, xmonad re-execs itself to reload its configuration. Good luck with that when it's also your display server...