Wayland was designed by people who didn't like the features of X that almost nobody used (X forwarding, for example). And X is still around for those obscure use cases, while Wayland can serve almost everyone with a much simpler and cleaner system.
Now, why Canonical decided to go off and write Mir instead of collaborating on Wayland development, I have no idea.
Now, why Canonical decided to go off and write Mir instead of collaborating on Wayland development, I have no idea.