I have longed for someone to ask this question.
Here is the reason:
In a multiplayer game, the game needs to send messages back and forth from the "Server" to the player. Nearly every part of Elemental was written this way.
Let me show you what I mean in pseudo-code:
This would be sent to an event queue which would then handle the message MSG_ATTACK_ENEMY which in multiplayer would send the message to the server and then send the results to the clients.
However, the tactical battles were not written this way. In tactical battles, they instead are: MySoldier->AttackUnit(pEnemy). It's all done locally, that is' on YOUR computer. The results are never sent off until after the battle is done.
In order to do multiplayer, we would need to convert this to a message based system.
Now, I'm not going to lie to you. I'm pretty pissed off about this. Everyone internally knows about this because we developed a world-wide virtual machine server system for Elemental. That is, when YOU create a game, your geographical location is used to tell a local licensed data center (We have them in australia, Na, and Europe) to create a virtual server for you and your friends to join.
Rewriting the tactical battles to be event driven isn't a massive thing but it would probably add a couple of months to development and because people like me (and you) are in such a minority, I can't justify that time until AFTER FE comes out and we see how it does. Multiplayer won't make or break the game. It's a "nice to have".
There are other, smaller, elements as well such as just having to constantly re-test that part of the game each new build which strains our QA resources at a time when Sins of a Solar Empire: Rebellion, which is a much more popular title, is using most of those resources. But the main thing is the missing message passing.
IF FE is a hit, I definitely want to add MP.