Other than server versions, Red Hat does an Enterprise Workstation and Desktop version.
Well, the "no good reason" for targeting consumer-oriented Ubuntu is because they get commercial support from Canonical. Red Hat doesn't even have a desktop GNU/Linux distribution, let alone offer commercial support for one, so it's hardly a surprise that a game development kit doesn't target RHEL.
Why would anyone consider writing graphics-oriented games for a server system? Do dev-ops have that much time on their hands?
Scientific Linux (a clone of RHEL by fermilab and CERN) is also used a lot in 3D Virtualization of "things". Unity would be daft to not make small tweaks to their software to target these platforms. I might mention it on the beta mailing lists because perhaps no-one else has actually tried.
It sounds like a few people still need a little bit more education and to do some more research before Linux ever has a chance of being popular with the masses. It will come one day though... ;)
What we need to do is get away from the belief that Ubuntu is the only Linux distro. As SimonForsman correctly said, it isn't too hard to write code that works on all existing Linux distros... developers just need to learn how to write slightly more portable code (like they have had to do for the various versions of Windows over the years).