It's likely a massive case of NIH. Red Hat, despite being significant contributors, seem to be a bit dicey on this kind of thing. One of the reasons Ubuntu didn't adopt gnome-shell and created Unity, has a lot to do with GNOME, and thusly Red Hat's, reticence. The thing applies to systemd's being chained to GNOME, which stands a pretty good chance of relegating GNOME 3 to Red Hat and Red Hat-ish distributions.
It's a real pity, too, because Red Hat has and does a lot of good stuff (GNOME 3, some weirdness aside, works very, very well). Package management and UI refinement just happen to _not_ be things that they're good at.
Canonical, to it's credit, has pushed the UI and user experience issue very well for Linux as a whole and raised everyone else's game as a result. Unity might not be their finest hour, but they are trying to meet the needs of most computer users and grow Linux in a way that most other vendors had not in the past (remember when UI development was "how blingy a theme can we make?", or KDE's tendency to cram as many controls per square inch as possible?), and it is improving, version after version.
Mint does it's own thing very well, too. What it is, though, ultimately, is a riff on Ubuntu for conservative technical people who already use Linux and for whatever reason aren't a fan of Canonical's design direction. That's fair, and a good thing, but it's not forward-thinking, and it's not a lot different from any one of a number of other distributions. It fills a niche, sure, but it's not the new Ubuntu, nor should it be.