Because the companies that sell those chemicals used to put your dog to sleep refuse to sell the drug to states for capital punishment (as they are afraid of liberals picketing them, etc.). The state could buy the drug and lie about why, but when the company found out the drugs were used on people, it would probably cease selling anything to the government thereafter, plus the bad PR of the state lying, etc. So, in the end, until a company is found that doesn’t care about the radical libs whining, states are dependent on small supplies from smaller dealers, that are commensurately harder to get and face supply issues. That’s what happens when pharma giants get too many libs in their corporate offices...
Also, has the drug been approved for use on humans by the FDA? Have tests been done? Side effects? Long-term consequences on health?