Oh I can answer that. In most states that have capital punishment, the method(s) of execution are prescribed by law. Now as to why the legislators do not accept other forms of execution such as Nitrogen Asphyxiation ... I attribute that to a lack of awareness or education on the part of the legislators.
Some good, capitalist U.S. entrepreneurs will fill the gap and start a business to produce the necessary drug. It’s the American way. You see a need and a void, you jump into it, and make money to boot.