Because he can drop the belt at elimination chamber. He wont be headlining wrestlemania. Its testing the waters. I really think thats what they'll do.
I'm too sleepy to do the footwork (KB if you're in here, help a brother out), but I'd be very interested in knowing how many challengers win the belt at the Rumble and then drop it at No Way Out/Elimination Chamber. I would imagine it's a very small number as it doesn't allow you to build much of a title feud for Mania.