I did some random research on the marine animals that's been featured on Reddit the past two days. Specifically the Great White Shark and the Ribbon Eel.

Found it interesting that both of them are not kept widely in captivity because they simply refuse to eat and starve to death.

Is there any particular reason why they do this?

I am not sure that there is a single reason in every case, but two factors are:

1) The captive environment may not allow provision of the preferred food which the species has evolved to eat. For instance, while sharks can be generalist predators of course,  it would not be legal or ethical to put live seals into an aquarium containing white sharks.

2) Stress. Chronic stress suppresses appetite. If we can't provide suitable environments for captive animals they will suffer physiological stress responses that suppress appetite, and immune function , and basically cause lots of other problems.