Nothing preventing you from writing a X11 server in something else either (and people have done so!). But fact is, most wayland compositors right now are either pure C or C++ (and I think the rest uses at least wlroots?). Many X11 window managers are written in non-c languages too and I don't think I am too far off the mark when I say that a decent fraction of wayland compositors would just be external window managers if there existed a standardized interface for window managers when they were written (I think some compositors have an interface for external window managers now, but is there a standard interface by now?).