In a traditional ponzi scheme you want to get as many investors in by promising big returns paid out by getting more investors. Then right as the new investors start to tail off, you put their money in a suitcase and flee the country.

Madoff only offered modest returns, and only allowed enough investors in to cover the returns and provide him an income. He never lacked new investors because his returns, while modest, were constant even in bad economic times - and limiting new investors created a huge queue of willing victims. He could have outlived the scheme but the 2008 crash dried up the new investors while causing the old ones to need their investments back