Well, it's not simply that it's the easiest, it's that being the easiest has led it to be the most widely-used by end users. (It's lost momentum to Mint since introducing Unity, but I think it's still probably reasonable to assume it's got a bigger install base than Mint.)
And its package system is widely compatible too. A package designed for Ubuntu should, generally speaking, work on Mint with no alterations, and on any other Debian-based distribution with only minor alterations.
OpenSUSE's still got a pretty big install base, and is well-supported. Its package management system is unique but it's based on RPM, so there's not MUCH challenge to changing a Fedora package to an OpenSUSE one.
(I would add that OpenSUSE's one-click install is the most goddamn beautiful thing I have ever seen and is one of the principal reasons I'm still using OpenSUSE despite some of the vexations I've had with it.) As it is, I think it's great that they put out an OpenSUSE package almost as quickly as they put out the Ubuntu one.
And while I'm sure the focus on Ubuntu is part of the reason I'm having trouble with it on OpenSUSE, it doesn't seem to be the only reason -- I've seen reports of Ubuntu users having the same problems I am. The bigger problem may be that, since Ubuntu's got a larger user base, they've got more people working on those errors and figuring out how to fix them -- and the same fixes don't work under OpenSUSE due to differences in package names, config locations, etc.
All that said: I don't blame Valve. Starting with the largest install-base of potential customers is the smart thing to do. I'm hoping that, once they've worked out major bugs under Ubuntu, they'll start looking at the bugs on other distros and, in the meantime, users will start to figure out fixes themselves and share with the class.
Of course, it would all be a fuck of a lot easier if the software were open-source, but you can't have everything.
