the wikipedia article on xmonad states that it is (was?) planned to
move to XCB "as soon as bindings are available". So, afais there are
quite recent bindings [1].
Is there a good reason why nobody has started on this yet?
Wikipedia is apparently way behind. It was started, and it was stopped because xcb turns out to require a complete redesign of xmonad. Likewise, there is no intent to port xmonad to Wayland; it could not be done as a port, but requires a complete redesign from scratch.
Other people are welcome to do such redesigns if they wish.