Because the executed's family still have a right to bury their dead, not poor him in closed casket. Besides, once the person is executed, it's considered they paid for their actions and deserve basic human decency.
Man. Retributive justice done in a potentially more painful way so that family members can do a ritual on your corpse. I hope our society continues becoming more enlightened. :D
Can you imagine the torture of waiting to be turned into a "meat cloud"? It's also probably prohibitively expensive to construct a "meat cloud device".