So how would the Gtk+ developers be able to ensure that it works under Mir? Mir is distribution specific. Same for an application running under Mir.
It creates lots of extra work for people who do not run Ubuntu. It will result in extra bugs as things cannot be tested. "choice", short-term benefit of trying to be first. Long term it seems really bad.
Gtk works under X, Windows and OS X. It already has a system in place that can support multiple backends and I am sure Canonical will be more than happy to write the Mir backend for Gtk. All Gtk devs have to do is to accept the patch.
It creates lots of extra work for people who do not run Ubuntu. It will result in extra bugs as things cannot be tested. "choice", short-term benefit of trying to be first. Long term it seems really bad.