He hasn't always been depicted so, the most common depiction of Jesus in late Imperial period was that of a clean shaven youth. Here are three examples, the first is from Hinton St Mary which is from Dorset England and dated to the 4th century. The second is the Ravenna Baptistery of Neon which is dated to the late 5th century. The final is the Basilica of San Vitale which is mid 6th century also in Ravenna and is the same church that has the famous mosiac of the Emperor Justinian and Theodora.
As to what caused the change in Christ's representations it's simply that beards came back into fashion among the rulers and Christ as the king of kings would be modeled after the kings who were around the artist of the time.
