Scientifically speaking, dead bodies make excellent compost material; and provide much-needed nutrients to plants in semi-arid grasslands such as Rohan, northern Gondor and Ard-galen. Since the Orcs are documented as having black blood, it is apparent that their hydrostatic chemistry is different from that of the Elves and Men from whom they are ostensibly bred "in mockery".
Red-blood chemistry is based on a combination of hemoglobin (as an iron-based oxygen carrier, hence the red color) and the molecular composition of sea-water, in which red-blooded animals evolved (primitive animals such as sponges used open circulatory systems to absorb nutrients directly through sea-water.) There are other, rarer methods of circulation found in nature besides hemoglobin.
A black-blooded family of species would presumably have a different blood pH and a much different salt balance in the tissues of the organism. It follows that the olvar, created by the Vala Yavanna, were not adapted to take advantage of the nutrients found in dead Orcs, which might be poisonous to them.
This does raise the question of whether Morgoth bred evil plant-life or fungi capable of decomposing Orcs and Trolls. Such would be necessary to overcome a resultant toxic-waste problem if long-term sustenance of (edible) plant life was a concern. Per Tolkien's Letters (in reference to Sauron's motives), "evil" in Sauron's realm is not deliberately wasteful, merely hostile to the natural ecosystem, which Sauron saw as inefficient use of resources (and a haven for Elves whom he hated and envied; Sauron would have greatly prized defoliants in warfare).
Morgoth however was often deliberately wasteful and nihilistic, so presumably he got a kick out of the steady nutrient drain from the ecosystem that would result in piling up millions of orc bodies on the battlefield that could not be decomposed except over very long periods of time (under high heat and pressure) into oil and plastics using abiotic chemical processes.
The fact that grass and moss would easily grow on a hill of decomposing bodies of elves and men in the desert or grassland, in comparison, easy to explain. The existence of flowers like Simbelmÿne which only grow on the mounds of tombs can be seen as a form of specialization, since those mounds provide turned soil, better aeration, deeper topsoil depth and rich nutrients.
Rocks, on the other hand, as used by the Huorns to bury the Uruk-hai, are equivalent to using a gravel layer in construction as a form of rip-rap; water passes right through them, preventing vegetation from laying down a nutrient-rich sediment layer which would then be subject to natural processes of erosion and plant cover.
There's also some illusion in HoME and Silmarillion to the order in which various plants were awakened by Yavanna, for instance flowers didn't "evolve" until the birth of the Sun. So it's unclear what sort of grasses existed in the Third Age of Middle Earth.
In our place and time, grasses aren't often the best suited to take advantage of decomposition, they rely on a thin layer of topsoil and often rely on worms and other beneficial organisms to do so. It is quite possible that red-blooded worms don't like to eat black-blooded Orcs, and so the cycle of decomposition is broken, preventing them from being composted.
As noted (and attested in the case of bog men), their bodies would be preserved as seen in the Dead Marshes, and gradually degrade under heat and pressure into carboniferous fossil fuels, creating phenomena such as marsh lights.
(note, this is simply a hard science justification, your mileage may vary.)