Like many people who play strategy games, we have a lot of ideas for cool features to have in a strategy game. They come up all the time.
Space mines, orbital bombardment, tactical combat, heroes, special agents, asteroid fields, special powers for races, and dozens I won't mention right now.
It's not that these aren't good ideas -- they are. I want many of those features too (except orbital bombardment <g>). For me, PC strategy games revolve around having good opponents.
A good opponent means either another human being who is decent at the game or a computer controlled opponent. For most of us, another human being who is at our skill level who is readily available to play a game that is going to last many hours is not a realistic option. That's the drawback on turn based strategy games (which isn't to say that we should shun having a multiplayer feature someday, only that it's not at the same priority as computer players).
The features I describe above could be added to the game in a month or two. Features are easy. Computer players who use them effectively are not. On paper, having a computer player who "uses" a feature effectively seems easy. But in practice, there's a lot involved. One mans genius strategy is another man's doom when played on a different map or in different circumstances.
So like I said, adding features is easy. You can create a bullet list of great features but in my opinion, if the computer players don't use them effectively, then it's largely a waste unless you have a really solid multiplayer component and then it's fine -- for the people who play multiplayer.
When we do the expansion pack, the features we choose to put it in will have to budget not just the time to implement those features but the time to write a computer AI that does the same thing and does it well. And that can be tricky.
And so next time someone wonders why "Feature X" isn't in the game, the answer may be as simple as "Because the time to have the AI use it well was longer than we could budget." If the response is "Game Y had that feature" then ask yourself - how well did game Y make use of that feature for its computer opponents?