I use Ubuntu because there is a StackExchange site, askubuntu.com, and because it's easy to set up and fairly reliable and I can always go to Archwiki if I want to dig deeper. And anyway, I run Xmonad so a lot of the gripes about Ubuntu's interface, Unity, don't really affect me. [1]
In the end it comes down to Ubuntu provides me with a better abstraction layer for support and problem solving over the top of Linux than the alternatives I've seen.
[1]: Edit. As that sort of interface goes, I think Unity is better than many, but it took some time fooling with it enough to be familiar to reach that opinion.
Yep. I tend to use Ubuntu Server and then dump i3 on it as my WM. The support from the community is second to none, so when I need answers they're always a Google search away.
Mind you, if I don't get answers I normally end up looking at the Arch documentation, which is phenomenal.
I used to do that with my XMonad setup, but at least with 16.04 and 16.10, I've found it more effective to go ahead and install Ubuntu normally, install Xmonad, and then use the session switcher to switch to XMonad. It seems to set up the NetworkManager and other similar such things more correctly, more easily.
I once went into an epic fight trying to make it so my normal user was able configure the network through network manager. Presumably someone out there must understand all that policykit and related project stuff really well, so well that nobody seems to feel the need to even remotely document it. (At least at the time.) I never did win; I just ran the nm-applet as root via sudo. With the way I'm doing now, it just works correctly, which seems to be the only way to get that stuff working at all.
In the end it comes down to Ubuntu provides me with a better abstraction layer for support and problem solving over the top of Linux than the alternatives I've seen.
[1]: Edit. As that sort of interface goes, I think Unity is better than many, but it took some time fooling with it enough to be familiar to reach that opinion.