And why New Vegas is using the rotting corpse of Gamebryo, instead of the fresh body of Onyx .
Most probably because they could reuse the assets and the gameplay would remain similar (instead of needing to redesign it from scratch). Also, from what it looks like, Onyx is more suited to a top-down view, but that may just be because Dungeon Siege III has been designed like that.
EDIT : But seriously, I'm really really impressed. The only thing that could be criticized are the ground textures, but there's still more than enough time to optimize the engine enough to fix them, and even with that blurry textures it still looks excellent.