Model S has 6,831 cells, not 6,800.
This is due to safety, reliability, cost and cost.
Cost: The capital cost of manufacturing equipment for 18650 cell is about the lowest it can get. This cell is larger than any other lithium-ion cell.
The cell is small and manufactured in large quantities, so the variation between cells is very low in comparison to other types of cells Nissan/NEC uses. These tiny differences are even more noticeable when they are combined into 69 parallel cell packs.
The cells are separated by gaps, which allows for cooling and reduces the risk of thermal runaway. You also get more surface area per unit of volume than with a prismatic cells, which helps with thermal management.