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.
People will always have a say to everything eventhough they aren't fully aware to the factual situation so why bother dealing with it? The individual that says bitcoin is a ponzi scheme or fraud or scam or any related tag to that are those who doesn't know how bitcoin and other coins are such a wonderful thing and can actually help you in financial aspect. They think that it is a scheme or fraud maybe because they experienced it or just brag that they don't like it. Still, bitcoin for others are a good asset you could ever have.