I think the biggest reason people fail is that they don't do enough research beforehand and don't use a reliable methods. We take it for granted that we can come on here and research reliable methods and learn the proper protocol for a method to increase our chances of success. Many people don't do that. They act on impulse and chose a method that has a low success rate, because they don't know better. For example, they think that they can take an entire bottle of pills - any pills- and it will kill them, or they can slit their wrists, because that is what they see in the movies, so they are setting themselves up for failure from the start .
The second biggest reason is SI. It is a lot harder to overcome your natural instincts which tell you to avoid pain and death at all costs than people who think it is .