Original: One reason that the scheme initially works so well is that early investors, those who actually got paid the large returns, commonly reinvest their money in the scheme (it does, after all, pay out much better than any alternative investment).
Simplified: One reason that the scheme initially works so well is that early investors - those who actually got paid the large returns - commonly reinvest their money in the scheme (it does, after all, pay out much better than any alternative investment).