The whole thing is exactly the same as what Activision has done with Call of Duty. Each Call of Duty takes 2 years to make. By having Infinity Ward and Treyarch make independent games they can release a Call of Duty game every November.
Now you have Bethesda which takes over 2 years to put a game out. So with two big series IP ZeniMax either had to expand Besthesda RPG division into two teams or farm one of the IPs to an external studio.
If New Vegas is successful I fully expect Bethesda to be just the Elder Scroll farm and Fallout to go over to Obsidian or another external studio for all future Fallout games.
Now you have Bethesda which takes over 2 years to put a game out. So with two big series IP ZeniMax either had to expand Besthesda RPG division into two teams or farm one of the IPs to an external studio.
If New Vegas is successful I fully expect Bethesda to be just the Elder Scroll farm and Fallout to go over to Obsidian or another external studio for all future Fallout games.