[qznc]

When Mir was initiated, Wayland's future was not as clear and little was working at all. Instead of talking, Canonical went and just did what they thought is the right way to do it.

You can consider this good (more doing, less talking) or bad (not invented here syndrom).

[bkor]

Actually GNOME shell has a Wayland compositor for around as much time as when Mir was initiated (not when it was announced). The future was also clear. Whatever reason Canonical had to start Mir, it is not related to Wayland.