It is also, ironically, because of a failure in the market.

A lot of drug manufacturers are reluctant to have "used by the Statesville Department of Corrections to execute people" on the Wikipedia page for the drug they are trying to to sell. Having your drug used to put people to death makes it harder to sell to people that want to stay alive. So even though there is an open market for these chemicals, it is harder and harder to convince manufacturers to supply that market. Even where it's not explicitly prohibited like companies operating out of the European Union.

Also, the whole killing people thing kind of antithetical to most medical codes of ethics. Even outside of bad publicity, there are moral considerations, most doctors do not want to put their name to an execution method, it kind of goes against the whole "do no harm" thing. This is why they normally have some random first-year nursing student set the IV. And also why so many executions have failed in spectacular fashion because they couldn't find a vein, or the IV pops out or whatever.