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.
We cant blame to call bitcoin a fraud or ponzi scheme as what they believe. It could be their judgement is based of the news, someone they heard or shared or their own personal experience. We cannot guarantee that bitcoin is 100% scam free. They used bitcoin for thier greedy but we can avoid them as we fully understand and knowledgable about bitcoin.