People oppose Mir for sound reasions
1. It mostly solves the same problem that Wayland is there to solve - rendering surfaces directly into hardware, proper input device handling without jumping through hoops caused by X11's arcane architecture.
2. Resources. Contributors are already hard pressed enough to migrate QT/GTK, display drivers and X over Wayland without adding another backend which does something similar but differently.
3. The differences in behaviour are quite vague (the Ubuntu wiki talks of 3d input devices and protocol agnosticism but doesn't explain why this is a big deal). IMO it is more likely that their business strategy dictates that they own the display stack and the technical reasons are excuses to support that decision.
4. Wayland is MIT licenced, the same as X.org meaning it's largely a slot in replacement.
5. Mir is dual licenced - GPLv3 or Canonical proprietary. It is not a slot in replacement.
6. Canonical do not accept contributions unless people sign an agreement which turns over distribution rights. The proprietary licence is there so they can easily release tablets or mobile devices with proprietary drivers or signed binaries while competitors are hamstrung by the GPLv3 which prevents those things.
So ultimately I think resistance is a combination of all these things. Companies like Intel are probably more concerned about licencing issues. Contributors are probably more concerned about the best use of their time and resources.