In a word, aeration.
Decomposition is enhanced by exposure to oxygen.
If you are composting lawn litter for your garden, it helps to turn the heap over with a pitchfork once in a while. In large commercial land-fill garbage pits, garbage is piled up deep and packed down with almost no aeration at all. The result is that the garbage decomposes only very slowly, if at all. You can excavate these pits years later and find mummified food waste in its original condition. Some landfill operators may have adopted new techniques in recent years that insure faster decomposition, but I do not know what the current state of the art is.