I cannot think of a better method than charcoal burning and carbon monoxide. I don't know why it's not as popular. Painless, guaranteed to kill, easy to perform anywhere, requires no controlled substance...
Like any method it isn't guaranteed, and there is a risk (albeit less likely and less severe by survivor tales) that there coud be brain damage if you are rescued at the last moment.
The big downside is the requirement for a small sealed space, which isn't easy for everyone to have (see Zanexx's post), the risk of harm to others if done indoors, and the time it takes to prepare, and also discovery due to smoke etc or being seen to light the charcoal and carry it inside.
I think if it was easier to just buy a can of CO it would be far more popular, but then not everyone choses their method based on ease, pain or success rate either, so other ways will always be popular. Depends on what you want to achieve and say in your last message :)