"Graft and corruption wouldn't have anything to do with it?"

Probably not. As bad as Kwame's abuse of the system was, how much did it really contribute to the overall cost of the system? Most of the corruption involved things like shaking down legitimate contractors to funnel money to Kwame's friends. You would have to find examples of contracts that never should have been awarded or contracts that were massively inflated to show examples of how that impacted the bottom line. DWSD has a huge budget and a million here and there in corruption isn't going to show up in the rates everyone pays. That's how Kwame and Co. got away with it for as long as they did.