Here is a blog post by a developer knowing what he's talking about and why Mir would have been
unsuited for Canonical. http://blog.cooperteam.net/2013/03/for-posterity.html
Also read later post of him if you are interested. They explain some architectural differences between Mir and Wayland
and why Canonical have chosen those.
But there are also one more big thing. Look at previous development by Canonical. It fast and may change direction
over one night. Look at Wayland development: It's better planned (and therefore slower) and lays much more energy
on having it right at the first time. If Canonical had sticked to Wayland I can guarantee that there would have been
much energy between Canonical and Wayland developers and probably resulting in a fork sooner or later.