A lot has to do with the nature of the old covenant made with National Israel at Sinai and the new covenant made with believers only.
Notes From Hebrews 8.6-9 | Feileadh Mor
A. W. Pink on Hebrews 8.
The republication of the covenant of works at Sinai:
In approaching the subject of the two covenants, the old and the new, it should be pointed out that it is not always an easy matter to determine whether the old covenant designates the Mosaic economy or the covenant of works which God made with Adam (Hos. 6:7 margin); nor to decide whether the new covenant refers to the Gospel dispensation introduced by Christ, or to the covenant of grace which was inaugurated by the first promise made to Adam (Gen. 3:15) and confirmed to Abraham (Gen. 17). In each case the context must decide. We may add that the principal passages where the two covenants are described and contrasted are found in 2 Corinthians chapter 3, Galatians chapter 3 and 4, Hebrews chapters 8, 9 and 12.
Pink, following Owens lead, views the old Mosaic covenant as a republication of the covenant of works made with Adam before the fall. He also finds evidence for the new covenant of grace
promised to Adam after the fall, fulfilled by the death of the testator Jesus Christ. [
Proto-Evangelium]
Commenting on Hebrews 8.6:
This more excellent ministry Christ is here said to have obtained. The way whereby the Lord Jesus entered on the whole office and work of His mediation has been expressed in Hebrews 1:4 as by inheritance: that is, by free grant and perpetual donation, made unto Him as the Son
The ministry of the old covenant was powerless, it never obtained anything and only looked forward to the promised Messiah.
For if that first covenant had been faultless, then should no place have been sought for the second. Hebrews 8.7
The covenant which is here referred to is that into which Jehovah entered with Israel at Sinai: see Exodus 19:5; 34:27, 28; Deuteronomy 4:13. Israels response is recorded in Exodus 19:8, 24:3. It was ratified by blood: Exodus 24:4-8. This was not the first covenant absolutely, but the first made with Israel nationally. Previously, God had made a covenant with Adam (Hos. 6:7), and in some respects the Covenant at Sinai adumbrated [adumbrated: To give a sketchy outline of; To prefigure indistinctly; foreshadow.] it, for it was chiefly one of works.So too He had made a covenant with Abraham, which in some respects adumbrated the Everlasting Covenant, inasmuch as it was one purely of grace. Prior to Sinai, God dealt with Israel on the basis of the Abrahamic covenant, as is clear from Exodus 2:24; 6:3, 4. But it was on the ground of the Sinaitic covenant that Israel entered Canaan: see Joshua 7:11, 15; Judges 2:19-21; 1 Kings 11:11; Jeremiah 34:18, 19.
Pink asks the question,
Wherein lay its faultiness? It was wholly external, accompanied by no internal efficacy. It set before Israel an objective standard but supplied no power to measure up to it. It treated with men in the flesh, and therefore the law was impotent through the weakness of the flesh (Rom. 8:3). It provided a sacrifice for sin, but the value thereof was only ceremonial and transient, failing to actually put away sin. It was unable to secure actual redemption. Hence because of its inadequacy, a new and better covenant was needed.
Yours in the Lord,
jm