To be fair to Ubuntu, they have to put a lot of resources into hardware support and the software installer. Apple doesn't have to worry about that, they have much more control over hardware.
... and you're probably right that user interface was the problem with Ubuntu up until Unity, too. The GNOME 2 desktop they used until then was functional and not totally awful to view but even nicely tweaked it can't match a well done KDE, GNOME 3, Cinnamon, etc... for visual style. Maybe Ubuntu really was our best chance and they threw it all away.
But blaming Ubuntu for the failure of Linux on the desktop is ultimately as pointless as screaming at Microsoft for ruining Windows with Windows 8. In both cases it's a product from a company that the consumer can't control. At least with Ubuntu people have forked parts of it in an attempt to make something better. You don't have that option with Windows unless you want to completely reverse engineer their operating system and build your own from scratch (good luck, ReactOS team, you've picked quite a dragon to slay).
I hope to contribute to Fedora or Debian eventually, but for now I'm too busy with other stuff - work, taking care of the kids, and wasting time on internet forums.