I also wonder why more people don't consider it instead of other methods that are more painful, terrifying, and take longer to complete. I think that the technical details of understanding what type of gas cylinder and flowmeter regulator to use and where and how to get them put off a lot of people, especially those that don't have a basic understanding of compressed gas and experience attaching threaded connections together. Much of the information on these topics in the PPeH, such as using disposable balloon helium tanks which should no longer be considered because the gas will likely be mixed with air, is outdated and incorrect, adding to the confusion. Once one understands that a sufficiently large cylinder of inert gas can be easily purchased online, at least in the USA, and affordable flowmeter regulators that attach directly to the cylinder are available, putting together the setup becomes very easy.
Some people may envision the gas cylinder being very large, heavy and thus difficult to transport, store and conceal if neccessary, not realizing that 20 and 40 cubic foot cylinders are less than 20 inches tall and are light enough for anyone to pick up and carry.
I have also read on this forum that some people are put off by having the bag over the head due to feeling claustrophobic, but because one will fall unconscious after a few breaths, it would only have to be tolerated for a very short time. Another concern I have seen some state is even after unconsciousness, the person may be able to remove the bag, but after many years of experience with this method by Exit International, they haven't stated any concerns about this happening. I believe them when they say one will quickly go into a coma after losing consciousness due to lack of oxygen to the brain, and body movement isn't possible after that. I don't have any medical expertise, but based on my layman's understanding of oxygen starvation, that makes sense to me. These are just my opinions, and everyone has to decide for themselves what they believe about how this method affects the body.
IMO if more people understood the information above, inert gas would be a much more popular choice.