This is actually a very interesting question, although the answer might seem somewhat broad and generic. It should first be pointed out that, although many of the Popes in our recent past have indeed been clean-shaven, historically there were also many Popes who did have beards.
Still, the reason why so many Popes and monks of the Western Church remained clean-shaven goes back many centuries. When a man or woman desired to enter monastic life, they underwent a ceremony in which their hair was sheared and/or their head was shaved. This practice, which began in the East sometime in the 4th and 5th centuries, was known as a “tonsure” (Latin for “clipping” or “cutting”). This tonsuring was meant to be an outward sign of forsaking worldly appearance and renouncing the love of body/self. The sheared hair became a symbol of that person’s “laying aside” or ridding the old self, together with all of their passions and desires. It was an act which represented the monk’s ability to undertake a new life in Christ. For this reason, after accepting this first monastic tonsure, individuals were no longer allowed to grow their hair for the rest of their lives.
However, as monasticism grew and reached its peak in the 11th and 12th centuries, a shift occurred in the East which prompted monks and nuns to not cut hair or trim their beards as a sign of asceticism – something clearly practiced by our Church to this day. Yet, though this change was made in the Eastern Church, the Western Church continued the practice of monks being clean-shaven and maintaining short hair. Therefore, since the Pope is also a monk, he generally abides by this tradition.
