Here's the problem:
They'd have to write the App from scratch. This wouldn't be a port of any of the existing codebases. It would have to be a whole new thing.
That's a LOT of work, and there's not really much of a 4X market for Mobile. Especially given that something complex like a 4X places significant hardware requirements on the phone/tablet, so it would limit the phones to which it could be sold. Let's not forget that Android is about 60% of the market, and the other 40% is iOS, and porting between those two is decidedly non-trivial. Doesn't help recoup your costs when your product can only run on 10% of the installed base.
Basically, you'd have to hire a whole new bunch of programmers, because (with all due respect to Brad and Co.), the demands of mobile programming are COMPLETELY different than for Desktop. The UI design is radically different, the UI coding different, the demands (and limitations) of the systems very different, etc.
It's also not a given that the experience of GalCiv is suitable for a mobile device. That is, the constraints of Mobile might produce a nice game, but it's highly unlikely that it will be "Galactic Civilizations" in any sense other than a superficial one.
It's never as simple as "just make it run over there".