The Pope's biographer Peter Seewald said that the Pope consulted one Person about his retirement from the Papal Throne: Jesus.
One rubric for understanding this type of a decision is the magnanimous missionary zeal of the saints.
"A strange thing in the sanctity of the saints is that when they perceive that their labours are frustrated for a time and bear no spiritual fruit they betake themselves to other places where the results are more palpable, for there is nothing to be gained if one stays in a place without reaping a harvest of souls..." The Anglo-Saxon Missionaries in Germany (The Life of Saint Boniface by Saint Willibald) p. 36. Sheed and Ward 1954.
Many have been reticent to convert on the urging of Pope Benedict. So he will continue to work on them and on the world from a higher place, closer to God and away from the world: the monastery.
Jesus Himself was unable to convert the people of His own town: so He went to the surrounding villages that did receive Him and worked His miracles there. (Mk. 6:5) And after a mere three years He had said all that needed to be said, according to the will of the Father.
Pope Benedict preached the Gospel among us almost eight years!