Porting is not simple. And target market share is critical.
Especially between mobile, desktop, and console.
In particular for Android/iOS, there are exceedingly few tablets able to meet the minimum specs for GC2, let alone GC3. Remember you'd need a minimum of 1.5GB of RAM, and something less than 10% of the tablets have that minimum. And, of course, spend a signficant amount of time figuring out how to completely replace the UI. Which doesn't even address the problem of whether or not any tablet CPU is sufficiently powerful to handle something like a 4X, especially since GC2 isn't super-threaded so individual core performance is still pretty important.
Frankly, Android and iOS are right out, since the number of tablets which meet the minimum requirements can't even be enough to possibly make back any investment (which is several engineer-years, I'd guess).
Possibly Windows Tablets might make sense, since they have a pretty good chance of making the min specs, and there's not a lot of UI porting to be done. But I'd have to see what the total potential market would be, and I don't have a good feeling about that.