The gallows has too many variables. Too short a rope, and you spend a few minutes asphyxiating. Too long a rope, and your head pops off. If the executioner make

The gallows has too many variables. Too short a rope, and you spend a few minutes asphyxiating. Too long a rope, and your head pops off. If the executioner makes a mistake, it's not humane anymore. If the executioner doesn't like you, he might intentionally "make a mistake". It's theoretically more humane, and harder to screw up, to OD you on happy drugs. (It also looks more peaceful when one of those drugs is a paralytic.) Oklahoma a few months ago introduced nitrogen asphyxiation as their backup plan. We know it's painless because a few people have accidentally entered nitrogen-heavy areas and didn't notice anything was wrong until they dropped unconscious (and were recovered before dying).