Why didn't the BOP function properly?

Today's NY Times has an in-depth look at the failure of the Blow Out Preventer (BOP) used on the Macondo exploration well. That failure resulted in the loss of eleven lives, the destruction by fire of the Deepwater Horizon drilling rig and the multi-million gallon spill that continues to flow in the Gulf of Mexico.

Lax Oversight Cited in Failure of Oil Rig’s Failsafe Device

It was the last line of defense, the final barrier between the rushing volcanic fury of oil and gas and one of the worst environmental disasters in United States history.