He is the creator of bitcoin. Just imagine what he will have to go through if he reveals his identity. He might be holding good amount of bitcoins and that can be a reason that to ensure his safety he does not reveal his identity.
I feel Satoshi staying completely anonymous up until now is more about keeping their influence far away from the network than their own safety. Satoshi did not want people to take them as a central figure in the network and take decisions based on what they do, i.e. if Satoshi were to be around and moved some of their coins, it would cause many people to also move their coins, that is the influence i am talking about.