Good thought. I had the same but didnt read the comments first or I would be agreeing with you.
They are killed by the anoxic events, which is just a fancy way of saying a lack of oxygen in the water. Reducing conditions. Therefore, most of the usual critters that eat dead fish (worms, etc.) aren’t around as they don’t like anoxia. Ever seen a swamp with that black nasty mud and murky water? That is anoxia. That is how coal is formed.