The most common assumption is Satoshi left because he doesn't want the attention and most likely had fears of government action since they have a history of going after developers of other tech related things that were then used for illegal purposes (various poker software creators have had legal action taken on them for websites using their software to cater to americans for example).

I think its no coincidence that he left right around the same time the CIA started asking questions and scheduling meetings to talk to bitcoin developers.

I also think he had the foresight to see some of the blowback bitcoin has gotten and didn't people to be able to character assinate bitcoin by attaching a single person to it.