"Mithrandir? Why the halfling?"
Why, indeed. He doesn't seem a likely choice. He's not particularly strong or wise or brave. We know in hindsight what he's able to accomplish; we can read to the end of the story. Without that omniscient view, though, he seems a very poor choice.
The choices Jesus made seem equally strange to us, don't they? For His closest friends, His apostles, he chose not the most educated or most politically saavy. He chose mostly fishermen. Among them, he chose Peter as the head, as the rock on which He'd build the earthly church.
"Lord? Why the fisherman?"
Perhaps it is because God can see the end of the story. He knew more than what Peter was. He knew what Peter would be. He saw the Rock, the first Pope, the martyr. He knew how Peter would end his earthly story:
"(W)hen you grow old, you will stretch out your hands, and someone else will dress you and lead you where you do not want to go." (John 21:18)