Pretty much. Also down to money, the proper way requires a full chamber, plus constant monitoring and replacement of all gasses in said chamber -- it can't be done with just a mask or helmet, that means thousands of liters of nitrogen per second to do it properly.
When done correctly, i.e. full immersion with proper removal of CO2, then it is humane. The current technique does not remove CO2 from the mask, meaning the victim chokes to death and knows they are choking the entire time.