They chose a mechanical non- biological method that could proceed without the medical community, again this isn’t out of benevolence, they were forced to act less cruelly because their hands were forced by the medical community.
And the old method was often misadministered so it was often as painful as they could make it. Link I have no doubt someone will find a way to make suffocation painful. Even if the method on the books is painless. The cruelty is/ was/ and always has been the point.