The mod team prefers to use canon designs unless they absolutely MUST design it themselves, such as with many of the EoTH ships. With the fact that they've got a fairly complete roster, and by far the best fighters/bombers in the game, then the Mediator isn't something they NEED. Yes, it would be nice to have, but it's not absolutely necessary for balance.
Plus, it wouldn't give them anything early. It wasn't designed until 17 ABY at the earliest (after black fleet crisis), so it would be available only in the same era as the Viscount...making it redundant.