I'm going to have to say that this was definitely not due to it being rushed. As Zoltan said, they most likely had this recorded and set up a good long time before the game was finished. I'm betting they only had so much money to allocate to voice acting, and the most important factions (read: the starting factions) got the most attention, because those are the most popular and the ones the most people want to play.

Now, I won't deny that this game did get rushed by the end, but the voice acting wasn't affected by it. Poor budget allocation, perhaps, but not rushing.