In each of the chapters in A Long Walk to Water so far, the author tells Nya's story first. Why do you think she does this?

There really isn't a "wrong" answer to this question, but here are a few possible interpretations:
Nya's story grounds the reader in the present, so beginning each chapter with her story could help the reader tie Salva's experience to the present day.
Nya's story is also more "simple" in many ways, which makes it shorter. Beginning each chapter with a short scene about Nya helps to invest the reader in the events to come. Similarly, Nya's portions often reflect a theme in the part of Salva's story that follows, which also gives the reader a "sneak peek" that keeps you reading on.
Nya's story also presents a bit of mystery, as we don't find out how the two narratives are connected until the end of the novel. Inserting a bit about her in the beginning of each chapter could also be a way to continue building the mystery in a consistent yet still enthralling way.