About 10% of Bitcoins were created early, before 2012, and have never been traded. If somebody ever finds the key of the early lost Bitcoins, they'll have a huge payoff, over a billion dollars. Speculation is that either "Satoshi Nakamoto", whoever he is, is holding onto them for a big payoff, or somebody lost the private key for all those early Bitcoins. As the years go on, the second explanation seems more likely.