I still don't understand that. Simply support ubuntu only, the other distributions will do the rest. E.g. the steam beta worked fine on archlinux and fedora from day one. And if they have problems you can either decide to look into it or ignore it because you officially only support ubuntu.
Firstly historically speaking Ubuntu has gotten famous only now.
Secondly, another distro fan is likely to argue <insert his favorite distro here> should be supported rather than ubuntu because of <insert a pedantic reason here>.
Agree on a standard that works for every one and then we shall talk.
The only reason steam is made specifically for ubuntu is because it uses apt-get to actually get steam. If valve put more effort into making it like every other linux application nobody on any distro would have trouble running it.
It's just another linux and everything can be ported from it if necessary. Even unity would work fine, once ubuntu's patched x.org and libindicator were on Archlinux.
As far as I know ubuntu has been the the most popular (for targetting software) distribution for several years now, so why not?