Normally i ignore discussions like this, but this irked me.
There is no legitimate reason why Galactic Civilizations III sould not be ported to Ubuntu Linux 14.04 64 bit via Steam for Linux and Steam for Mac
1) Galactic Civilizations III is 64bit. How many Linux/Mac home users are already on 64bit.
2) Galactic Civilizations III uses - as far as i can see - the Granny, Miles, and Bink libraries van RADTOOLS and possible other libraries. For the Mac and (a limited number of distributions of) Linux 64 bit was only recently introduced (Miles) 07/04/2014, Granny (12/01/2012), Bink (Pre release: 11/15/2013, first stable: 01/31/2014). And quite a few of the recent bugs squashed are for these versions. So there are hardly any game developer who can have experience with these 64 bit versions. And blindly thinking it works on the windows version, so it should work on any of the other variants is not the reality of the this world.
3) Various libraries have demands for the supported compiler (IDE) and other things. On windows for example the Bink library now works under VS2012, on Linux it compiles with GCC 4.8, this means that you will need to make your sources and resources compiler/platform agnostic. This will take lots of time. VS solutions are not Linux makefiles. So double your software-project overhead because it needs to be kept in sync.
4) Debugging various platforms will required different skill sets: experience with different debuggers, memory dump inspection/interrogation, log file analysis. This will probably mean additional programmers, support staff.
5) Execution security on Linux is not standardized between the various linux distributions, which means SELinux will be probably need to be disabled. Are you happy about this? A lot of people have it by default on and don't even know how to switch it off.
6) Not all Linuxes are equal. For example Kubuntu is the 36th most used variant. Maybe you are lucky and will it work for you because it is derived from Ubuntu - the number 2. Because i do not believe SD is going to distribute the game in compilable source form.
7) Doubling/Tripling your license fees while still in the development phase is not the wised business strategy.
So i see several reasons why there are legitimate reasons for currently not supporting multiple platforms.
Even other A-class games that do support multiple platforms, there is always one primary platform and once that is up and running, the other supported platforms follow thereafter. And this is not just the initial release, but also any patches and other things like DLC, expansions and so on.
If a Linux version would arrive in the future and i wouldn't have to buy a second license (on top of the windows version) just to run it, then i would happily play on one of my Linux systems.