There are many, many ways to lose a cryptographic key. There are none to restore one. There's also the idea that Satoshi thought he was doing something altruis

There are many, many ways to lose a cryptographic key. There are none to restore one. There's also the idea that Satoshi thought he was doing something altruistic and purposely destroyed the keys so he wouldn't be tempted later on... which, if true, he might be regretting now.