Because it's irreversible, and people contemplating suicide are often being impulsive or illogical, yet feel as though they are being completely logical. Maybe they're chronically sleep-deprived, or having a vitamin deficiency or drug-interaction. The problem is that they will think they are making a decision of their own free will.
This is why things that affect your very thinking are so insidious. Think of sleep deprivation or hypoxia. Ever watched a video of someone trying to draw simple shapes while not getting enough oxygen? The crucial point is that they think they are okay.
When I was feeling suicidal it seemed like the only logical solution to a set of unsatisfiable constraints. However, none of these were particularly hard constraints. As a relatively privileged person I was ashamed to reach out for help, because I felt I was letting everybody down. Don't be ashamed. It can do weird things like convincing you that it's better to kill yourself than inconvenience your self and others by e.g. quitting a job, or failing some courses.
To make a coding analogy -- sometimes your client / management etc. tells you they want X and Y and Z. The set of solutions to X and Y and Z is empty. However, if the constraints were relaxed, X and Y is solvable. Y and Z is also solvable. Maybe they didn't really want X anyway. This closing down of options seems to be a common pattern, however it's often false.
Very few people are actively being tortured to the point where death is the only logical escape. Typically, there's more of a spiral inward where more and more options disappear until only one seems to be left. Typically there are so many steps along this path that removing a few constraints or realizing that they're not really there can make all the difference.