1. Satoshi vanished just about the same time Hal Finney was diagnosed with the terminal disease ALS. There is no any other good reason that Satoshi retired from the project at that particular time.
It's been speculated that satoshi disappeared around the time the CIA became interested in Bitcoin. Gavin Andresen wrote that he was invited to present Bitcoin to the CIA. Satoshi stopped being active around that time I think. One might say that he wasn't comfortable having the CIA look more closely into his work so he handed over control to other developers and left.
Whatever his reasons for disappearing were, I guess we will never find out.