As a former U.S. Navy nuclear submarine sailor, and current, and past, employee of a BP contractor on Alaska"s North Slope, I'd say ruveyn's and ZerOnetgain's opinions are closest to spot on. In short, regulators failed to regulate and enforce, and BP's focus is largely on the bottom line for this quarter; a phenomenon common to all public corporations.
For some perspective, look up Piper Alpha, the Texas City refinery, or the '05 spill on the North Slope. Also, BP's own website gives insight into the focus on cost cutting.
I contend one of the reasons for the extensive use of contractors, is the ability to point the finger at someone else when things go wrong. Safety is perpetually rammed up my backside at work, however, on the "energy octagon" I fill out before performing a task, there is no blank for "piss poor engineering".