There are a lot of possibilities, satoshi could've forgotten about that 1 million bitcoin and never bothered to find it, that seems ridiculous but that can still happen. We will never know because we do not know who is behind the curtains of the account satoshi. Best thing we could do is honor satoshi's wish to be anonymous and cherish the brainchild that is bitcoin.
Extraordinarily too ridiculous to forget an invention that's been getting a lot of attention lately don't you think? A lot likely for Satoshi to be either dead or simply refusing to touch the funds for whatever reason.