When President Barack Obama signed the Dodd-Frank financial reform bill on July 21, he capped a year-long legislative battle to stop $1.8 trillion in global financial writedowns and losses from happening again. The law forces derivatives trading onto exchanges or similar systems, creates a consumer-protection agency to monitor loans and lets the government unwind companies whose collapse would destabilize markets. Obama didn’t mention one culprit in the financial debacle during remarks at the Ronald Reagan Building in Washington that day: credit ratings...