Why could God not make the new covenant earlier?

Scripture indicates God could not make the new covenant until the prior covenant had run its course. The Abrahamic covenant still needed fulfillment—Israel as a nation in its land under God's government—and the law covenant from Egypt had to be completed. Only then would Israel's spiritual condition permit the new covenant.

Les explains that the new covenant could not come until the old one had run its course. The Abrahamic covenant still had to be fulfilled: Israel would become a nation of people living in their geographical area with a super government—God Himself in the person of Christ the Messiah, their King. When that becomes a full reality, Israel's spiritual condition will change. Only then will God make a new covenant with the house of Israel and the house of Judah. This new covenant is not according to the covenant made with their fathers when He took them by the hand to bring them out of Egypt, which was the law. That law was harsh and a burden.

Jeremiah 31:31

Behold, the days come, saith the LORD, that I will make a new covenant with the house of Israel, and with the house of Judah:

Why can't believers in the Age of Grace be under the New Covenant?

The New Covenant is made specifically with the house of Israel and the house of Judah, not with the Body of Christ. Since God makes it with Israel alone, believers in the Age of Grace cannot be participants in that covenant; they only enjoy its benefits, such as the cross, resurrection, and Holy Spirit, without being in a covenant relationship.