In early times, the privilege of papal election was not reserved to the cardinals, and for centuries the person elected was customarily a Roman priest and never a bishop from elsewhere. <answer>To preserve apostolic succession</answer> the rite of consecrating him a bishop had to be performed by someone who was already a bishop. The rule remains that, if the person elected Pope is not yet a bishop, he is consecrated by the Dean of the College of Cardinals, the Cardinal Bishop of Ostia.
Why did the ceromony of consecrating a bishop have to be preformed by someone who was already a bishop?
To preserve apostolic succession