We're talked about Tactical Combat before here.
There's a couple of major drawbacks:
(1) Assuming that it's a turn-based Tactical Combat (e.g. "chess with spaceships!"), resolving a single combat can be VERY time consuming. So time-consuming, in fact, that it's worthless for multiplayer. All multiplayer games will have to have auto-resolve turned on, and bypass TBTC completely.
(2) A basic TBTC setup isn't terribly hard to implement, but that's unlikely to satisfy the people who really want a TBTC thing in the first place. As your wishlist shows, people want a whole bunch of additional "realism" features, and that's a whole lot more coding and and graphic work to take into account. Don't forget, you have to have all those "realism" features added into the Auto-Resolve portion too, otherwise, AutoResolve Combat and Manual TBTC modes produce radically different outcomes, and that's something that will *very* much piss off the userbase.
(3) An AI which will fight effectively in a TBTC is VERY difficult to write. The AI for the AutoResolve combat system isn't much more than a number cruncher, and doesn't really need to take into account much more. An AI for a TBTC needs to be *at least* as complex as a good Chess program. That's several orders of magnitude different in effort.
(4) If you get into 3D combat desires, well, that's virtually an entirely different game wrapped inside of a 4X. Think about the effort it takes to write something like Homeworld, then imagine having to implement maybe 50% of that to get the kind of 3D TBTC people want.
Basically, it's not simple to add. I could see it being a future Expansion Pack (it's far too complex for DLC) and people having to cough up $25 for it.
But the bottom line is that it's not an intrinsic need for a 4X to have a TC mode. Plenty of people will buy GC3 without the TC feature, which means it's not necessary for the initial version. More people would buy it WITH the TC mode, but I'm pretty sure it's not economical to wait for TC to be implemented and delay the whole game to get that extra audience, especially since most of them will buy GC3 WITHOUT the TC mode, and separately buy a TC pack later.
Money, dear boy, is a factor. 
(HoMaM3 isn't a 4x, and Civ 5 doesn't have any sort of TC mode - the bonuses are for position and such, but the combat resolves just as GC does it)