Seems to me these places get hacked at a magic number that fits with a lifestyle change.
As much as everyone hates Mt.Gox because of the cost to put money on there and the loss of anonymity, it seems like they have the best methods on there.
Don't forget that we don't know everything. It's perfectly possible that other exchanges are being hacked (broadly speaking) for less than "lifestyle change" amounts, but the operators choose to eat the losses themselves and keep operating (cf. BTC-E a few weeks ago).
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.