I think in a lot of cases it's because there have been so much suffering (the reason for ctb) and a lot of people would like the last couple of minutes in their life to be calm and peaceful. Personally I don't really know any methods that would be peaceful for me. Except powdered short acting barbiturates which I can't source (No, if you plan to send me a "source", don't waste your time) So even a method like Nembutal (IV-fluid) wouldn't be peaceful for me, because even though the chance of failure is small it's still possible. So I prefer a method that can't fail (Like full suspension, which is 100% if done correct and not being found too early) But with a very small risk of a decent amount of pain even if done correct, but I would be willing to take that.