My guess is that he just lost the keys in the usual hard-drive crash or forgot-the-password sense. This was a research project, remember. Did Satoshi have any reason to think the early coins he mined would become this valuable in real currency terms?

Having lost the keys, he now has zero incentive to reveal himself. Consider how many people would never believe him if he came forward and said his keys were lost. It would suck to have everyone think you're a billionaire when you're not. The only rational option for Satoshi may be to cry himself to sleep every night and take his secret to his grave.

He (or they, didn't some folks think it might be a multi-person persona?) might have intentionally destroyed the keys, to avoid this temptation for whatever reason.

Well, no-one would surrender to the Dread Pirate Craig.