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.
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?
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.