Because we as a species perceive time to flow linearly from a beginning point until a consequential point. For that reason, it comes bvery natural to us to read about things from the beginnings; how they started, who caused them, who was involved. Then through the actual events; how the war looked like (just an example), how many casualties were there, where was it fought, etc. All of this is then followed by a ending sequence where we learn about the aftermath.
This chronological thinking about events is often applied in high schools when teaching history classes because it's very easy to follow and is more understandable to the pupil.