Basically, Ubuntu phones were using click packages (predecessor to snaps) back in 2011 and 2012, with snapcraft shipped in January 2013, whereas xdg-app (later renamed to Flatpak) was started in December 2014.
Another thing to consider is that Canonical is orders of magnitude smaller than RedHat, and they do have a problem getting the community involved, but part of that is the size and limited time their developers have.
Now, even as a long time Ubuntu fan, I'll probably be switching to Debian just because I dislike the snap upgrade model.