My best guess is that he thought 99% of people would take what he said at face value, and the other 1% would be ignored as nutters, but he judged it wrong.
This is close to my theory. He thought he would get the media on his side, to write articles about how he is Satoshi, and then it would go into Wikipedia. Because Wikipedia cares about what journalists say, not some bitcoin basement guy who can mathematically prove he's lying.
...and then he can say to the Australian tax authorities "see I mined these Bitcoins legally, I didn't get them through money laundering". That's a suggestion on a BBC article (http://www.bbc.co.uk/news/technology-36191165). But even then, he could have said "I mined them" without saying "I'm Satoshi".
This is close to my theory. He thought he would get the media on his side, to write articles about how he is Satoshi, and then it would go into Wikipedia. Because Wikipedia cares about what journalists say, not some bitcoin basement guy who can mathematically prove he's lying.