Very good information, Nitrous. I learned something. Maybe others have, too.

One of the main reasons for choosing lethal injection is because the procedure is generally considered to be less traumatic to the witnesses than other methods like electrocution or firing squad.

Nurses and doctors generally aren't allowed to perform it due to Hippocratic Oath and/or Declaration of Geneva. That leaves it up to others with questionable training, which may explain the success rate compared to other methods.