No matter what we think and guess for his/her/their reasons for getting out, nobody will be able to verify that. So, we can tell things and speculate what could be the closest reasons for getting out. One of it is being dead and that's the very first thing that we can think of when he's no longer visible.
But as said, no one will ever know the truth about it. This is going to be remain one of the mysteries that we will never know even in the future because of too many people claiming they are satoshi without even proving it.