Cultural saturation, accessibility to impulsive people, ease of use. People who are looking to die generally want relief from pain, rarely more of it.
People generally don't know the unpleasant things that can happen in an overdose, whereas violent deaths are scary and sometimes even hard (guns aren't available everywhere, people who want to jump might fail because of survival instinct, hanging is associated with the death penalty and pain, everyone knows drowning sucks—actually I still tried it and was traumatised a bit by it). Supposedly disfiguration of body is also a concern, and of course so is involving other people. The users here who research still want N after all?
Other poisons are known to be painful for the same cultural reasons and also seem less pleasant and peaceful. They're also less readily available to mind.
It's worthwhile noting that this varies greatly from country to country. Some methods are more popular in some places because they're more obvious and available and then people learn you can ctb that way. Sure enough, pills used to be much more effective before less lethal alternatives to old medicines were developed.