The main reason for the mummy is that the place where the corpse is buried is very dry, and most of the time the wind is strong, and there are substances with strong water absorption in the soil, which absorb all the surrounding water.
The graveyard is a shady place, where nothing grows, suddenly it becomes dark, and the sun can't be seen all the year round, so the body can't combine with the sun and the sudden acidic organic matter, and it can't be decomposed.