This might be a really obvious thought, but it just occurred to me that the difference between writing fiction and nonfiction is that in fiction, you construct a world and a story, and your job is to create the illusion that there is stuff going on outside the boundaries of what you’re showing, or that the world is somewhat messier than the way you’ve depicted it.
When you write nonfiction, you do the opposite. The real world is an inherently messy place, and your job as the writer becomes trying to construct this illusion of order through what you choose to include (or not), how you choose to present things, and how you order or juxtapose different events and/or ideas.