The answer from God’s word:
“The things recorded in the Old Testament are Jehovah’s work in Israel, and that which is recorded in the New Testament is the work of Jesus during the Age of Grace; they document the work done by God in two different ages. The Old Testament documents the work of God during the Age of Law, and thus the Old Testament is a historical book, while the New Testament is the product of the work of the Age of Grace. When the new work began, these books became out of date—and thus, the New Testament is also a historical book. Of course, the New Testament is not as systematic as the Old Testament, nor does it record as many things.”
from “Concerning the Bible (1)” in The Word Appears in the Flesh
