Big exchange like Gox falling down is a big thing. 744,000 BTC stolen is also a very big thing. Who says this can't happen again with one of the other big exchanges?
Not only
CAN it happen again, it almost certainly
WILL happen again.
It is your responsibility to make sure that you don't purchase a promise from a person or organization that is not going to deliver on that promise. If you choose to take a risk trusting an entity to deliver on a promise, then you'll experience the consequences of that decision.