What makes his so-called stash of bitcoins worth the effort of trying to go after him for a cut any more than how wealthy any one of the other multimillionaires or billionaires in the world happens to be?
This is pure speculation, but that's all we have to work with right now.
Nakamoto has worked as a scientist on several secret military projects. This has given him some insight into the ongoing US government research into time travel technology. He must know that by attaching his own identity to the creation of bitcoin, he would become a lucrative target for criminal time travelers. By keeping his identity anonymous, he was protected against time travelers visiting him on the day he created the algorithm and having it stolen from him.