The problem with most punishments like these (along with the death-sentence) is that if you convict someone wrongfully; you can't recant the punishment and apologise. Jail-sentence is horrible enough, but it is survived, most of the time, and then, if you have been sent in on bogus evidence, you can just be let out. Scarred, but alive. Hard to dig up a dead guy and say "Sorry, we made a mistake." Same with castration.