There could be a few reasons, and usually they are not good reasons: maybe they lost their private keys, maybe they simply forgot the bitcoins they mined/purchased a long time ago in an old hard disk that they thrown away. It's not nice to say because probably we're talking about relatively young people but they could have died. I find difficult to believe that otherwise someone wouldn't touch his bitcoins for such a long time, not even to moving them to a new wallet or address?