For me, I don't want to use a firearm (of which I own several adequate to the task) because of the trauma it would cause my family to see me with my brains blown out. Firearms make a pretty dramatic mess of your head. I know my family will want to see my corpse, and if I can spare them the distress of seeing me so maimed, I would rather do that.
Other people I have spoken to are resistant to using a gun because of PTSD, or are simply afraid of guns --just as other people would never consider jumping because they're afraid of heights.
Speaking theoretically, it is not necessarily so easy to get a firearm here in the US as many non-US residents believe.
Each of the 50 states has its own set of paperwork and legal requirements, some of which must be completed before the purchase, some at the time of the purchase. Some of those states, such as here in Washington State, make it ridiculously easy to get a gun, but others, such as Massachusetts or New York, are very, very restrictive, to the point where you might as well be in Europe.
One universal disqualifier from owning a gun is if you have been involuntarily committed for mental health reasons --which you'll soon find, from reading around this site, includeds a significant percentage of the forum. Unless you want to learn about "black powder" muzzle-loaders, which are not restricted, that takes the gun option off the table right from the outset.
There's also the fact that firearms are expensive; the cheapest option, a shotgun, will typically cost around $200 new, which can be a lot of money, and a basic handgun will easily cost more than twice that. Used guns are cheaper, but if you are not already part of "gun culture," the process of buying a used gun can be both confusing and daunting --even more so than for buying a new gun. Black powder guns, and their operation, are another labyrinth entirely, so despite their being less expensive than cartridge guns, the learning curve is daunting. Then there are the costs of the paperwork, tax, etc.
So yes, the method is very reliable. But not everyone has access to it, and it is not appropriate for everyone for any number of reasons.