It's probably because looking at the material makes it all real. Suicidal ideation is, after all, by definition a "fantasy". Actually setting up a contraption that will kill you takes it from thoughts and rumination to something that will actually end your life once you use it.
I have had my suicidal ideation drift off plenty of times, but never because I've entered into serious planning and/or obtained materials - I've never done that. Usually, it's been because of an improvement in my mood/motivation/outlook.
I've swung back now, and I know what you mean about how "confusing"/jarring/whatever that can be. "How could I feel one way just the other day, and a different way now?" It doesn't seem "right". But if anything, it underscores how fickle our emotional states can be. There are a few users on the forum who display this cocksure, grandiose idea that they have it all figured out - they see right through this risible charade that is existence and bla bla bla.
David Foster Wallace put it in more simple and modest terms - a suicidal person feels like they're in a burning building. They don't want to jump, but it's unbearable to stay in that building. I guess that some days, our inscrutable brain has us waking up in a different building.
All of the fancy talk about existential/philosophical angst or nihilism or what have you, it tends to smack of someone trying to convince themselves.