To understand the strangeness of the New Orleans cemetery culture. We must return to the beginnings of the city itself. You see, for the entire length of its existence, New Orleans has known death. Just a few short years after the colony was founded, it was flattened by a hurricane, bringing ruin and destruction. For years after, the impoverished colonists saw their numbers reduced by the grim reaper.
The city was always wet then, as it continues to be today. The original site of New Orleans, which is the French Quarter today, had a water table just beneath the soil. The land sloped back from the river toward Lake Pontchartrain, falling quickly below the level of the sea. The question soon arose.... where would the colonists bury the dead in such water-logged conditions?
The highest area in the region was along the banks of the Mississippi. The natural levees there had been created by years of soil being deposited by the river’s current. This was the first site chosen for burial of the deceased. During floods (which came often) though, the bodies of the dead would wash out of their muddy graves and come floating through the streets of town. Obviously, this was considered a problem.
...
For all but the indigent, above ground tombs were the rule. The reasons were obvious as the wet ground of Louisiana caused the graves to fill with water. The coffins would often float to the surface, despite grave diggers placing heavy stones or bricks on the lids. Such conditions did not appeal to those with the wealth to be buried in style.
http://www.prairieghosts.com/deathway.html