He probably forgot his key to his wallet
He did. He said so himself here:
I no longer have the keys that were referenced in my initial emails with Satoshi. That's why the remaining coins are unspent. At the time bitcoins had no value at all and nobody else was using the system, so I didn't bother backing up the wallet and eventually lost it. Back then there were no forums, no markets, no exchanges, no usage at all as far as I could see and Satoshi did not seem to have any interesting in marketing either. Months would go by and nothing seemed to happen. It was just an interesting science project on SourceForge, one of many, which seemed destined to sink into obscurity just like Ripple had. I don't beat myself up about it. I did pretty well out of Bitcoin in the end.
https://www.reddit.com/r/btc/comments/6t2ci2/never_before_seen_mike_hearn_satoshi_nakamoto/dliizv6/What does he have to gain by revealing his identity.
It is not a question of what he has to gain but rather what does he have to lose? Regardless of who Satoshi really is, Satoshi was likely employed somewhere in the field of computers/coding/programming/IT. This means that if it was Google, IBM, Samsung, Microsoft, or even some small time software company, they all would have a legitimate claim to ownership of bitcoin's source code. Unless you think Satoshi was unemployed, this is the most plausible reason he/she had to hide their identity.