Probably because of accessibility and illusory peacefulness compared to how gruesome hanging is portrayed in media. The actual peaceful suicide methods (opioids, barbiturates) are illegal in nearly every single country, and I think most people aren't willing to take the risk. Being hellbent on chemicals that are legal and more easily accessible does leave scant options. This is probably why this method is frequently talked about here, a lot of the "classic" non-narcotic means of poisoning oneself were banned ages ago.
Even if you're in a country that has laxer drug laws, the penalties for possession of strong opioids for recreational use can be quite harsh. A lot of people here have been sectioned or dealt with the cops before due to being suicidal and probably don't want a repeat of that if they slip up. My ideal method of choice for myself out of what is accessible in this day and time would be CO2, but it's difficult to pull off or conceal the equipment. I have no idea how many Korean celebrities and idols manage to do it in very cramped, crowded cities in non-clandestine hotels.
I chose SN for myself because I've tried hanging before and it was just too painful. The consequences of messing up and failing a hanging seem to be way worse, and less reversible. It's easy to abort an SN attempt within a reasonable time window, an antidote exists if one changes their mind at the last second or can't bear the pain. With hanging, you have a very real chance of sustaining brain damage from oxygen deprivation if anything goes wrong and you're found/fail the attempt.
I definitely don't think SN is peaceful or pleasant, but for the reasons stated above, I chose it for myself over other more crude/violent methods like hanging, drowning, train, etc. To me those are last resorts and I wouldn't want to do something like hanging if other options existed.
