I would imagine degree of toxicity, or 'taste' might have something to do with it. The Koran I kept years ago ate everything - even Palythoas. That guy was a monster, but one of the coolest, and easiest fish I've ever owned.
I agree with the previous post about fish eating only stuff you don't want them to. Like the Asfur that cleaned off an entire head of tricolor Symphyllia in an evening, after ignoring every coral in the system, Symphyllia included, for years. Needless to say she graduated to the FOWLR tank the next day.
DJ