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.