Which is why it's horrifying that CO2 asphyxiation is the "euthanasia" method of choice for everything from lab animals to livestock. We've managed to select the one gas that inflicts excruciating pain.
At a guess, I'd say we selected that gas so that, in the event of a safety failure, human operators would notice they couldn't breathe, and flee. It's a pretty good reason.
Assuming the humans are always in a ventilated area, though, nitrogen seems better along several dimensions.
