Identity
We don’t know why exactly he chose to keep his identity a secret, we guess he just doesn’t like fame. But it can’t be that simple, there must be a more philosophical and deeper reason for him to keep his face hidden. Well, the best reason we can think of is all because of his art.
As you can see, he believes that art should be the thing we celebrate and not the artist. That means he took the power away from his identity and status and focused it all on his work. We suppose that he was trying to make a statement, saying that we don’t know what art really is.