1. no one else wants to use Mir
2. Canonical doesn't care about the compatibility or feasibility for other distros, Mir will be designed to work for Ubuntu only
3. Everyone else is moving to use Wayland, and Ubuntu will be the only one using Mir, making Ubuntu entirely separate and incompatible from the rest of the Linux ecosystem.
Ubuntu has abandoned the way of the desktop Linux ecosystem. It may have a Linux kernel, for now, but it is moving away from being a part of the ecosystem. Soon, we will have separate software for Ubuntu and other Linux distros, just like we already have separate software for Android. Just because something has a Linux kernel doesn't mean it's part of the desktop Linux ecosystem.