The body is weird when it comes to breathing. It doesn’t measure one of the critical gasses. 3 things particularly send the body into a breathing panic.
Rising CO2 (via blood acidity)
Water in the airways.
Resistance to inflating the lungs.
Water boarding is particularly evil, since it creates just enough of the last 2 to trigger a full blown drowning reaction, but is light enough to not actually be dangerous. This lets the questioner hold the victim in that zone, without permanent physical harm (but massive psychological harm).
Nitrogen hypoxia doesn’t set off any of those triggers. This makes it particularly dangerous to some workers. They don’t realise anything is wrong until they pass out.
Also, to clarify. I am massively against the death penalty. It’s both cruel, and not particularly effective as a deterrent. It’s also no cheaper, in practice, than life imprisonment. However, if it is going to be used, it should be as humane as possible. Nitrogen hypoxia is about as humane as it can get.