Sorry I am late to this but crvz has it right...
Their diet may be even more specialized than he poorly understood Linckia. So in large tanks with a lot of high quality LR, they are more likely to survive.
Unfortunately they are often quite small, and so people put them in nano tanks. And they live several months, which people think is the "normal" life span.
It is hard for us to understand but indeed it can take a very long time for these animals to starve. This is why no one should claim "success" unless they have kept them 12-18 months in a system. A few months tells us nothing about success.
Seastars have a variety of defenses against disease. IMO, most die of acclimation shock and starvation primarily - diseases would be secondary to this. Many seastars (and echinoderms in general) do not tolerate synthetic salt mixes are the variations in parameters encountered in our systems. This causes significant damage and stress.
Other explanations, like "exposure to air" are also ignoring the primary causes of trauma - osmotic shock and starvation.