I think because the exit bag strategy requires more materials, some more difficult to hide, like a big tank of helium being one. Plus cost. Also alot of manufacturers are cutting their helium now to prevent any "accidents." There are also alot of failure stories, even while being unconscious, people find their bodies survival instinct have cause them to rip the bag off their heads without even knowing. SN is pretty available, fairly cheap and easy to hide up until the day needed. Just my opinion, although the exit bag does seem like a very peaceful exit if done properly without any interference.