Well, they say that when paying for a job you need to choose two from: fast, well and inexpensive.
For suicide methods you could imagine a similar triangle with painless, fast and easy.
For instance inert gas would be relatively difficult, but once set up fast and painless. Hanging is easy and relatively fast, but can be very painful. SN can be difficult to obtain, but is supposed to be quite fast and relatively painless. N is very difficult (to obtain), but definitely fast and painless.
So, if you need something that's fast and easy then hanging it is. There's a reason for the popularity (or notoriety) of this method.