Because if it happened once, it can happen again. Was it a cockpit fire like in Cairo, a new issue, bomb, suicide, etc? Are you willing to budget another bazillion dollars and 200+ lives for the next time.

Those flight data recorders capture thousands of parameters from every system on the aircraft. They're not just voice recorders. The data on the flight 370 recorder matters more then anything else in preventing a reocurrence.
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Hope for the best and prepare for the worst.

The object in life is not to be on the side of the majority, but to escape finding oneself in the ranks of the insane