All bitcoins are created from mining rewards. As the number of coins has increased (and more people started mining), the difficulty goes up and less coins are p

All bitcoins are created from mining rewards. As the number of coins has increased (and more people started mining), the difficulty goes up and less coins are produced. These 'whale' entities either ran mining operations early enough, or (more likely) took a huge bet on the direction this was going and decided to accumulate - like the Winklevoss twins did - by buying as many as they could from miners.