What's so great and unique about this game? The long and intricate storyline, and the multiple ways to solve quests, which can result in some of all of the quest-givers dying, permanently. Neither are conductive to multiplayer.
And it's in no way at all as simple as slapping on a new menu. The game's graphics engine is optimized for single-player. Its coding is not designed to deal with lag. Designing for multiplayer and single-player are entirely different things, and multiplayer is by necessity a shallower and simpler experience.
