I figure it would be a lot easier to create scene-managment in C# and XNA, at least from what my experiences tell me, but hell that was quite a simple octree scene managment system, and thus not really efficiënt.
I also wonder why there are no big projects made with XNA.
Maybe people still believe that C++ and directX will give them a great speed advantage (ok, the speed will go up a little, but it's not even that much anymore). I personally think that projects with C# and XNA would be more managable.