They expected to be better off the moment they invested in but the idea of bitcoins is not as they think, hence they will tag it as a ponzi scheme or fraud.
Different people have different opinions about bitcoin. According to some it is useful and profitable thing for them. Where as according to some people its is scam and it does not a profitable thing. Many people like this mind set also considering bitcoin as pornzi scheme. Bitcoin volatile nature is the biggest problem of it. Because of this its price fluctuate time to time. And many investors loss their investment and consider it as bubble or pornzi scheme. According to them it increase market price is and all other benefits is just for show off.