The problem is the companies who make the drugs do not allow them to be used for executions.
Here's something I haven't understood about that. It's my understanding that the most common death for people's pets in the USA is now euthanasia. Someone is making a drug that causes mammals to die an easy death and is allowing large quantities of it to be used for that purpose. Since the objective is death, who cares if the drug hasn't passed the regulatory hurdles that would allow it to be used to treat humans? I'm opposed to the death penalty, but as a practical matter, being unable to figure out how to humanely kill people in 2015 seems idiotic.
Understanding is always the understanding of a smaller problem in relation to a bigger problem. -- P.D. Ouspensky