Sorry to hear this. They are one of my favorite fishes.
Assuming water quality, diet, space, and environment are top notch, and eliminating a disease or pathogen, then there leaves the possibility of this is as much captivity as the fish can handle.
We rarely know how the fish we acquired was handled and there is a possibility that in handling the fish was just too stressed.
Often though, it is water quality, diet, space, or environment which may appear to be okay, but which there has been a chronic problem that has finally caught up with the fish. Not all fish react immediately to an issue -- not all issues are acute -- some take the stress until the fish is spent.