Because Ubuntu is still the closest you get to "it just works" for Linux Desktops. (Not claiming you actually get there, just close).

I think this is quite a misconception. I've used Ubuntu for years and it basically broke on me with every major version upgrade.

I have never ever hat a Fedora system bail out on me in that manner and I have multiple PCs. My Dad and my Grandma's friends also use Fedora and they have yet to manage to break it and they are allowed to install software.