The reason for the delisting of a token is to first look at its volume because an exchange only delists a token when the volume of that token loses volume at the speed of a storm. When investors realize that the project team is cheating then they quickly sell tokens their token. which reduces the volume and causes the project to be removed from the exchange.
beside about trading volume usually exchanges team look at project developtment progress. If after several months or year there is no important developtment exchanges will warn dev team first before decided to delist them. If any die project token still in market it could harmed investors , so its good prevention and protection step from exchanges to their users.