Even when conditions are right, it is still incredibly rare for an organism to be fossilized. Scavengers can eat the edible parts, and scatter the rest. Soft parts can decay, and erosion can destroy the rest. Even if the organism is protected by being buried quickly, later erosion, deformation of the rocks, the action of ground water, and many other things can prevent preservation. I have seen estimates that even with rocks that contain lots of fossils, for every fossil you find, there were millions of organisms that were not preserved.