Could be two reasons. They were a scam, or the support, volume, etc made it a dead coin.
If we examine it for concrete reasons, there are actually not only two, but there are several reasons that make the project eventually die, firstly because of a lack of money, secondly they want to deceive a lot of people, third because the team's performance is not in line with existing ideas in road maps, etc.
Cryptocurrency startups can have much higher profitability than regular companies. Not all of them "work", but this is normal. In my opinion, if 3 out of 10 projects are sold, this is a good indicator. The main thing is that those that sold out, showed a return on the entire portfolio and returned the entire amount of investment with a profit.