From what I understand, it was all crazy at first, "omg we already have some kind of multiplayer support!", and right now I can't find anything, except stuff about it from 2009. Couldn't find a straigth recent answer otherwise.
Seems to me like no one could properly figure out how to do it and I'm sure its way more trouble than its worth, considering the MMO will make that obsolete.
If I remember correctly, their excuse was something like... they wanted to make a highly customizable and replayable game in a very short time period with a very small budget, and then, if it was well-received, create a multiplayer version later, which is what the free-to-play action-MMO version coming out within the next year or so (my guess) is supposed to be.