Some people think he just left so that bitcoin wasn't under his control/guidance and it could left to be truly decentralized. Personally I think he died.. I don't see how any human could leave their wallets untouched if they had that many bitcoin in. Unless he/she was already wealthy 
if he/they was already wealthy?
after death someone inherits something. it is as a rule.
If satoshi was already wealthy that may explain the willingness to leave their wallets untouched. No one would inherit them unless he gave them the private key, which if they do not want the money for themselves they're not going to pass on to their next generation on death. Inheritance is also not a rule. People can simply forego their things when they die and no-one inherits them.