There are many species that have rather specialised requirements. The causes of their untimely deaths is unknown, but that is because the species are not widely studied, as they rarely have any commercial value. Many common tropicals were impossible to keep a century ago because few field trips had really studied the environment. Once the importance of simple things like stable temperature, pH etc., were discovered, many previously difficult fish became easy.
With these particular horrors, I suspect there is something missing from their captive diet that is essential for their well being.