A very astute observation! The papacy changed under JPII who came in quite young. In his youthfulness, he reached out to the world, visiting a multitude of countries, expanding communications worldwide and initiating the World Youth Day events. Those initiatives set a pace that is challenging for a more senior cardinal to follow. In contemporary times, the pope must be able to "tweet" and reach out to the youth of the world.
Pope Benedict gave it his best and, I believe, recognizes that now is the time to select a successor who can meet the challenge.