As you open the first page of the book and begin reading, you will see that there is interaction with the narrator right off the bat, the narrator being death. Death introduces Hans Hubermann right away and goes into background information about him. Death likes to interact with
the reader and give hints and ideas of things that are about to happen. Death is the only character that you could have narrate this book since he already knows everything. It also lets death connect with the reader since he is telling a story from the eyes who is not human, therefore he has no reason to lie.
Death likes to talk about his own life a lot in the book. As you read more into the book, you will take notice of this. Death introduces WWII and the Holocaust from his eyes, since the book revolves around these two events. Death does not tell these stories in order, but instead moves around on the time of line of events that happened. This is what gives the the foreshadowing of Himmel street being bombed.
While many could argue that the book revolves around Liesel and her viewpoint, Death is still telling the story. Death makes the story flow in more of a compassionate way, which makes the reader forget that it's really Death telling it. The book does not make Death out to look like a evil looking figure. It makes him seem like an actual person, since it explains that he takes souls as gently as he can and shows a compassion for chocolate. This is why the author chose death to narrate, since death seems to connect with humans.
It is easy to see why Death is the narrator of the Book Thief. If death was not the narrator, the story would not be able to flow as nice as it does. Having just Death be the narrator is best since it gives Death the ability to tell the whole story without contradiction. The story would not be the same if Death was not the narrator.