Indeed as Brixity said, there is monumental difference CB and LB in terms of risk and the burden of the task (LB is way more simplistic in task, area to cover, and amount of players you have to keep track of, if you lose your man as LB it may indirectly lead to goal, but with CB, its every single time a direct goal chance).
You rarely see young players excelling directly as CB at all (people like Micah Richards are big exceptions), its a very demanding position that warrants for experience and smarts, if you dont have the physical qualities enough impose yourself. Which again, Micah Richards has too, in if he is off position, he is fast enough to bounce back, same with Chiellini who is strong and quick enough to deal with the task when he isnt on point positionally.
Criscito while not just being young and inexperienced, I saw in Azzurrini too before he came to us, was way to weak physically to impose himself in any way in vital sitautions like that. Too easily bullied off the ball.