Because burning a body is inherently disgusting. Try it with a small animal, see how it is. Commiting someone to the earth is far less awful, far more natural, and just generally better.

In concept, cremation is lovely. In practice, you need a bone grinder, a cremation chamber that requires you to break the femurs of the dearly departed to fit them inside, and massive filtration to prevent the entire area from smelling of burnt corpse. Burial requires a box.