Captonian, 4000 meters depth is likely the chief reason. Look at how long it took to reduce the Titanic. Two years would not be enough time for the little critters at that depth to make much headway consuming the bodies. There are few sources of energy down there for life. So for the most part it'll be few, far between, and somewhat sluggish, I suspect.
(Did I guess right auv-ee?)
{^_^}