It is death penalty opponents that are against it.
The closest death penalty opponents have gotten to eliminating it is getting drug companies to stop supplying the traditional lethal injection drugs. With that, executions have ground to a near halt, as alternatives do not work as well and has been challenged.
But nitrogen is easy to obtain and painless. So if it gets used, there goes the need for drug companies. So opponents have taken to outlandish and disingenuous arguments claiming it is dangerous and painful despite plenty of evidence to the contrary. Once it is used for the first time, the experimental argument is dead. So they are desperate to avoid breaking the seal, as it were, on nitrogen asphyxiation.
Of course vets are against it. They have no difficulty getting the necessary drugs, and they have no desire to add new equipment for it.