Because it can't be withdrawn and Mt Gox will probably disappear soon, taking people's coins with it. People buying it are buying the coins at way below market

Because it can't be withdrawn and Mt Gox will probably disappear soon, taking people's coins with it. People buying it are buying the coins at way below market rate, essentially gambling that Mt Gox will eventually allow withdrawal (presumably they have other bitcoin investments too), while people selling are sensibly trying to get what money they can out before the company folds.