Why not 5…why not 12? Is 6 feet recommended non-stank level for decomposition?
And, of course, the old joke is, John Holmes(an Ohio boy!) is buried 6’11" deep.
It is a theft deterrance. If they dig them six feet deep it will take most of the night for a grave robber to dig out the coffin, if the grave robber finally gets the hole dug, it will be daylight and he will be seen.
If they dig it much deeper it is hard to get out of the hole.
Graves need to be dug be below the depth to which frost penetrates the earth (frost line) in order to avoid the upward movement of their contents by the action of frost heave. Frost heave occurs when water freezes beneath a buried object and expands. This forces buried rocks, coffins, skeletons etc. towards the surface. The frost line normally reaches 3 to 4 feet beneath the surface in the northern U.S. so a six feet grave leaves some room for exceptionally cold winters.