What if..
.. MtGox got hacked like a year ago, with BTC stolen, and they were too chicken to announce it, instead thinking: "Well, we make good money don't we.. maybe if we just keep going, then maybe we could earn back the money we lost."
But after a year, everything has just went over their heads and they are insolvent.

That is the only reason that I can imagine of. Other then the theory that Gox is operated by an flock of drunken emus.