There are many reasons why a project can die. The first one would be that the project was always designed form the start to just be a quick cash grab for the team, so they just hopped on the current hype that is happening at the moment like Defi, NFT or play-to-earn games then they quickly created a website imagined a roadmap that is based on nothing and just launch the project on Uniswap or pancake swap and hope that they find a few poor fellas that ape in. After a few weeks it will be probably become obvious that the team is not delivering what they promise and the project will die.
To one of your other questions: I don't think that a project that is currently in the top 10 or 20 can really completely disappear anymore because those are all multi-million-dollar projects already so the only way that they would disappear would be a epic scam or a massive event like a hack or a critical error in the blockchain code or something.