In the beginning, buyers buy into a ponzi scheme because of a story that the creators are telling about their scheme. As time goes on, people buy in because the early investors are making a ton of money, as the value of the asset begins to skyrocket. Later on, people buy in because of previous hype, even though returns are actually beginning to level off, people don't notice. In the end, returns start to go negative, and the asset plummets, because nobody else has any reason to buy the asset anymore.
With bitcoin the theory goes: In the beginning, buyers buy into bitcoin because of the vision and idea of a decentralized currency made by an anonymous hacker on the internet. As time goes on, people buy in because the early investors are making a ton of money, as the value of the asset begins to skyrocket. Later on, people buy in because of previous hype, even though returns are actually beginning to level off. In the end, however, due to the properties of the currency, and its penetration throughout the population, people continue to buy even though there is little profit to be made anymore. It has been shown to be a better long term store of value than fiat currency, but with similar liquidity, unlike other assets with higher returns such as bonds and real estate. It can be easily transferred around the globe and is highly divisible, making it useful for payments large and small, avoiding complex VISA networks and regulatory red tape. Due to its enforced scarcity, it isn't possible to create any more of it, and so it maintains its high price rather than simply being overproduced back down to a lower valuation.