the cause of this happening is not much different from what happened to a failed business in the real world. the lack of attractiveness of the coin, lack of financial support, and developers who do not have long-term planning are the causes of many failed coins on the market.
Based on the fact that the cryptocurrency market is currently not regulated, there are more fraudulent projects appearing here. Therefore, a lot of developers set themselves the goal of getting rich quickly at the expense of other investors, after which they abandon work on the project.
They are doing this to scam people, they see opportunity on this market since many are able to make money even if the project is failed, so some are trying to become more greedy and create their own project even if they have no plan to continue