It's also perfectly possible that other exchanges are being hacked in "lifestyle change" amounts but they continue to operate in a combination Ponzi/Flying Dutchman mode, accepting deposits and cheerfully reporting "balances" that are pure fiction, hoping that someday they'll make enough "profit" to earn their way back to solvency, or just because they can't bring themselves to admit that things are broken. The deeper the lies go, the harder it is to come clean.
If an exchange was doing that, some people would probably hold them up as a shining example of a well-run exchange that was impervious to hacks.
I'll point out that Mt.Gox is a Japanese company. The culture over there is even less inclined to admit fiduciary mistakes than it is in the Western world. Look at TEPCO for a prime example. If the underbelly of Mt.Gox were being eroded away by security breaches, they might not say a word.