The reason for people to believe in bitcoin being a ponzi is the volatile nature. Its up one minute and the next minute it is on an steep downward slope. In simple they have no insight into whats happening around.
Though they are wrong it seems there is a little truth to it. People invest when the value is low and they wait for others to invest again and the price starts to go up and they immediately sell to the extent that it goes downhill and then they start tuto whole thing again.