If you think that Sarbanes-Oxley has been effective, why weren’t the financial institution problems uncovered under audit, before they collapsed in 2008?

Has Sarbanes-Oxley succeeded in cleaning up corporate financial scandals? I note that restatements, a sign of malfeasance, seemed to be declining until recently. 
If you think that Sarbanes-Oxley has been effective, why weren’t the financial institution problems uncovered under audit, before they collapsed in 2008?

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