A Ponzi scheme is a fraudulent investment operation where the operator, an individual or organization, pays returns to its investors from new capital paid to the operators by new investors, rather than from profit earned through legitimate sources.
Bernie Madoff was able to keep the scheme going for so long because of his personality. People invested their money with him because he seemed to be like a trusted person. The people didn't catch up that Madoff what the one doing the scheme because he was "investing" their money in stocks and making the people believe that he's using their money for good.
A Ponzi person is very influential and is very good with words. They act like they can be trusted by people so they basically convince people and puts them into traps and takes money. They are respectable. He lures people and pretends to be a "business" person.
After the warnings from Bernie Madoff, the SEC did nothing because Bernie Madoff was good at hiding the fact that he was the one doing the scheme. Bernie Madoff had family behind this scheme and well they just helped him to keep luring people into investing so he can keep stealing money and pay for the investors and everything he had to pay. He had a good personality to trick people and so the SEC did nothing after so many obvious warning that Bernie Madoff was the one that was doing the scheme.
Bernie Madoff did the scheme maybe because he just wanted the feeling to have money. He just did it since he thought he could probably get away with it just like the other people that tried to attempt to do a ponzi scheme.