I think it's obvious what happened. When he first posted the original paper, there were lots of small cryptocoin systems out there. If you read the message board you can see comments from other people working on other systems, but not with a blockchain.
Later on, bitcoin blows up and he decides anonymity would be better, and eventually leaves the project.
His brother said he suffered a stroke, and it may be that he's no longer as smart as he used to be. It's also possible he forgot the passwords to his crypographic keys. That might be one reason why he might not want to come forward - if he'd lost the keys he could be extremely embarrassed.
Baseless speculation. Satoshi tried hard to be anonymous from the very beginning, he never gave any personal info despite people asking from early on and always used anonymizing techniques. Furthermore, thinking that a system like Bitcoin could come out of the blue from someone who never published before any work related to cryptography/electronic cash (and not a single blog post or mailing list mail) is beyond naive.