It's been discussed for a long time. Last I heard, the devs were still of the opinion that tactical combat is tilted too heavily in favor of the player, who will always take advantage of whatever chinks and flaws are in the combat system. So that's why this is a strategy-only game. Also, having just one AI engine for strategy means the devs can focus 100% of their time and attention on that one set of code, instead of dividing it up into two modules. At least I think those are the major arguments.

Speaking for myself, I wouldn't mind seeing tactical combat if it was done well, but not if it meant the quality of the strategy side of the game would suffer. Stardock isn't a big company. I'd hate to see too many resources diverted to something that might turn out to be mostly eye candy, if it wasn't just outrageously good. I've seen that happen to other strategy games.