Megan McArdle has a good thought on the ability of Madoff to operate for so long.
It's actually easier for someone committing outright fraud to slide under the SEC wire, because when you're making up financial statements out of whole cloth, it's not hard to make them conform to SEC regulations. Similarly, Stephen Glass and Jayson Blair and Jack Kelley were able to get away with their frauds precisely because they fabricated things out of whole cloth. If you misquote an actual person, or misreport something that really happened, there's a strong risk that you'll be caught. But manufactured sources never complain that you spelled their name wrong or twisted their words.
Hitler commented on this idea.
Make the lie big, make it simple, keep saying it, and eventually they will believe it.
The great masses of the people will more easily fall victims to a big lie than to a small one.