It was at that point, when Moses had given up all hope of power and glory, that God called him to be the Leader for his people.
It does seem strange why God waited so long to call Moses. Why not at 20? or 40? when Moses was in Pharaoh's court. Why not at 50? or 60? when Moses probably harbored a deep desire to go back to Pharaoh and kick his arse? Why at 80? When Moses was old and decrepit?
But that's the thing. Moses wasn't ready until that moment. He just needed to spend time in the wilderness, doing seemingly mundane things, letting go of his worldly desires.
Maybe if God had called Moses before when he was still in Pharaoh's court, Moses may have simply staged a court revolt, and usurped Pharaoh's throne, and ignored the call to lead God's people into the Promise Land.
Drat its 4.40am now...