A few reasons.
1. In prisons ,they don't use real doctors,they basically pick someone to be the "prison doctor"
2. They don't just use Pentobarbital. They also use something to paralyze and stop the heart of the person. Absolutely no reason whatsoever to do this other than save the viewers from seeing the death seizures of the body. It's definitely excruciating if the sedatives haven't kicked in , and maybe even after.
In short, they're idiots who don't know what they're doing. The prison system in the US is a catastrophe.