Because it's hard to get cash out of Mtgox and the price reflects that fact (few people cash out dollars on Mtgox, but I think more trading goes on). That's al

Because it's hard to get cash out of Mtgox and the price reflects that fact (few people cash out dollars on Mtgox, but I think more trading goes on). That's also the reason you can't just buy low and sell high, the cash out time is 2 weeks, and a lot can happen in 2 weeks.