Who is Arthur? Let's Discuss

Something I've been wondering for a while is "Why is there so many different variations of King Arthur?" Most myths or stories the characters stay about the same throughout different medias but with minor changes to their roles not their history or personalities. This is not the case with Arthur however as stated in the readings. For he has many different renditions of him from his personality, background, looks, etc.  as well as many different tales from folklore, myths, literature, and romance. So how did this great debate of who King Arthur truly is really come about?