I'm completely oblivious here, so this might sound stupid; but is it possible that he lost access to the wallet before bitcoins were actually worth anything? I.e, he just didn't bother much with backups or security because losing a million bitcoins would be like dropping a 50 cent coin.
I think this is a very solid theory if you think of satoshi as an average developer. How many of us have created an app that had promise but let it die due to apathy/boredom/etc
I believe, based on the white paper, that his creation was ideologically motivated. He believed in what he was making, and would have taken every precaution possible.
There's always the chance he didnt think it would take off and let his Bitcoin stagnate and get lost. But I don't believe it is so.
There is a quote of him saying something like he would never destroy a private key. And the fact that his disappearance happened right before everything really took off, but I clearly remember there was a very tight knit group of people who absolutely believed in the cause.
And he handed development off so carefully, I can hardly believe he would have just neglected something in my opinion he personally believed would be world changing.
I think it is much more reasonable to assume that he fears for his life and about hacker attacks and that is why he takes every precaution possible to conceal his identity. People have killed other people for less money than a million bitcoins.