Probably because it seems so easy and painless, just tie a belt to a doorknob, sit down and you'll be dead in no time. Celebrities allover the world are doing it, so it must be super effective too, right?
But if this forum or any other forum where people discuss suicide prove one thing is that this is not really that easy or effective. There are posts like "I tried hanging but I failed" - every single day. "I can't do it, I can't pass out, it hurts too much, it doesn't work" etc.
Personally I don't see the appeal of a method where you can back out so easily. I'll be doing full suspension.