I tried to use partial suspension hanging to kill myself about 15 years ago, as a teen. It almost worked, was totally painless, and I fainted quickly. No pain, no pressure in my head, nothing. I passed out so quickly, that I didn't feel any mental discomfort before. I woke up, and clawed the rope off my neck, terrified. The fear is an automatic response. I have no idea how it failed; I passed out completely, I did everything correctly. I had studied how to do it for half a year before trying; I took it seriously. It almost worked, but it didn't. Every day, I wish it had. Better to die as a teen than a burned out loser like me. I've kind of lost my faith in the method up until now, but will try again soon because my life is still unbearable, and I have no reprieve, none. I'm sick of this, and I'm sober so I can't drink or drug. Can't get a gun because of involuntary hospitalizations. OD doesn't work, I've tried and it's bullshit. So hanging it is.
To answer your question directly, there are plenty of reasons not to, even if you have to dig so low that the reasons relate to the act itself, distinct from positive circumstances in your life. Pain, risk of failure and the complications with that, family, friends, et cetera. Fear of hanging, fear of gunshots, fear of falling from heights, fear of judgement. Those reasons seem to scare pretty much everyone. Or maybe long-stemmed glasses, a movie, and a pleasant dream in midair? Anything can be a reason not to.
I am sorry you're in pain. This world is a nightmare.