Why do you suppose the author did not reveal what happened to Liesel in the years between her reunion with Max and the time of her death?

Zusak was concerned about his characters at a given point in time. He skips ten years to allow hindsight and reflection before the two characters meet. War and peace time are two very different entities. There is both a sense of maturation and internal reckoning in both these characters which makes the reunion all the more poignant.