It's because they dont follow the rule to the bounty campaign they join thats why they dont get their pay.
Some ICO not pay the bounty. There are many scam ICOs. Now, most bounty not escrowed by manager. The manager leave the sheet after he sent to the dev. the payment from dev. it is developers who not pay the bounty. They forger that bounty job is not free. Hunters spend time and money for internet connection. The developers may think they will succed their projects without bounty. That is why some scam developers not pay bounty hunters.