It's because during the early days of Bitcoin, there were a lot of didn't understand it and so they claim that it was similar to Ponzi scheme or even to Tulip Bubble. But we have both disproved that already as Bitcoin can't be compare to them. I mean if the tulip mania, it was burst and never comeback, as compare to Bitcoin wherein every 4 years we will get into a point wherein there is a new all time high. As for Ponzi scheme, it's obvious that we are trading here, others sell, and then on the other part of the globe, there will be buyers that will win and make some money, and on the other hand those who sold might have their reasons and they could comeback again and re-buy. Which is very far and different from a definition of Ponzi or pyramiding scheme.