Although the Bible does not explicitly explain why Chronicles calls Keturah a concubine, the book’s purpose helps clarify the emphasis. Chronicles was written to establish the lineage of the covenant people, tracing God’s chosen line.
By calling Keturah a concubine, the writer highlights that Sarah alone held the unique covenant status of Abraham’s primary wife and that Isaac alone remained the heir of the promise.
Chronicles is not merely giving family history; it traces the covenant line from Isaac through the patriarchs and ultimately to David, emphasizing the Davidic covenant.
First Chronicles focuses especially on the lineage leading to David and the establishment of the kingdom, and Second Chronicles continues that covenant theme through the period following David’s death.
In other words, Keturah may have been Abraham’s wife, but Sarah remained Abraham’s covenant wife.