For a lot of reasons, but mostly because the traders are increasing prices everyday instead of selling it lower than what they spent.
I think because of it's price, really. Bitcoin is making a lot of people rich with just waiting for the price to pump. With that people are holding bitcoin so they can too be rich with just holding their bitcoin, but I think if you start in bitcoin couple of years ago, you are all millionaires now. Other than I think because of it's features like the pseudo anonymity and the decentralization.