Because if there is no economic return on a fish, and no Quota Management System standards applied, there is no guarantee the marlin would be kept in suitable conditions, iced and packed correctly, to ensure food safety. 

Creeping death. Once commercial fishers are permitted to land ‘dead’ marlin, those numbers will increase and the argument will then change to let them keep them for sale, to get a return on their effort. 

This means there will be an incentive for fishers to target marlin.

Allowing commercial fishers to give away dead marlin will put this magnificent species at risk of exploitation for little to no gain for the public and the marine environment.