The simplest explanation is that he is dead, killed by a jealous government when Bitcoin was just starting because it was suddenly new and associated with Wikileaks.
That requires a rather not-simple explanation for how someone posted as Satoshi during the Dorian thing.
Satoshis gmx email account was hacked. With this email adress the hacker was able to change the account passwords of sites like the p2pfoundation, where he posted that message on. Months later he wrote an email to a trusted member of bitcointalk, saying that he has proof of Satoshis identity and that SN is still alive.
Hacker was "doxxed" by a bitcointalk user called "greaterninja", but since there is still a bounty on hackers "head" I don't believe the doxxing hit the right person. "Greaterninja" enjoyed the fame for beeing "the one who doxxed the hacker of the
[email protected] account within 30 minutes", but he never explained why the big bounty is still up there. If he doxxed the real hacker, he truly deserves the bounty. He doesn't seem to have ever claimed it.
This is how you can read it in this forum, but it does not necessarily mean, that all of it is true.