Yeah, I still wonder this too, and I've been here for a few months. I think most prefer it due to the fact that it takes very little willpower compared to other methods such as jumping or hanging. There is also a big fear surrounding overdoses here, since they are regarded as a non-method thanks to
a post made by a late member which advises against inefficient methods, including over-the-counter drug overdoses (ironically, the user notes in the very same post that specific combinations actually CAN be lethal).
The bottom line is, most people here don't have the willpower or interest to really educate themselves and ascertain what the easiest/best possible method for them is. They just want to do the bare minimum to get out quick. SN is physically painless, but it's extremely uncomfortable regardless. Headaches, fever, air hunger, nausea...
There's also the issue of documentation. There are a lot of suicide methods out there, no doubt some of them much better than SN, but people don't have the knowledge on how to carry them out.
SN is one of the most documented methods on the site. People feel more confident going through with it, because they have exact step-by-step instructions on what to do. Suicidal people don't want to experience any pain, and the prospect of making mistakes is scary, considering the potential consequences. They want to die so their consciousness stops.
Personally, the "smartest" suicide I've seen here is a guy who agreed that SN sucks, and instead died using research-grade opioids and benzos he purchased off of the grey market, causing a very pleasurable slip into unconsciousness, and then death. They're not very strongly regulated, since they are literally too pure to be safely used for recreational purposes (though they are VERY expensive). This will be my method too.
As for the origin of the 'catch the bus' idiom, I have no idea. I think it's just a relic from the old days of the forum, when the community was much smaller. Some member must have used it and it just happened to catch on.