I'm not sure where I stand in relationship to Mir vs. Wayland. I hope they are both successful. I believe Canonical has done Linux a favor. If not for the success of Ubuntu, we probably wouldn't have Steam on Linux now. Gaming is becoming better on Linux, as are gfx drivers. Just a few years ago, both of those were in terrible shape on Linux.
The desktop computing paradigm is giving way to mobile. Already, mobile devices outship desktop devices and the trend will only continue. Desktop devices will never die, but mobile computing will become the dominant force in the future, and Ubuntu see's that and is pushing Linux (whether the rest of OSS world is ready or not) in that direction. I don't know why they couldn't use Wayland, but for whatever reason, they felt they needed something different and thus created Mir. I'm happy to see the rapid pace of their progress.