It's easy to understand if you study the subject.
Who is Abraham's seed according to the NT?
“Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Christ.” (Galatians 3:16) (KJV 1900)
Why did God give the OT Law to Israel?
“Knowing this, that the law is not made for a righteous man, but for the lawless and disobedient, for the ungodly and for sinners, for unholy and profane, for murderers of fathers and murderers of mothers, for manslayers, For whoremongers, for them that defile themselves with mankind, for menstealers, for liars, for perjured persons, and if there be any other thing that is contrary to sound doctrine; According to the glorious gospel of the blessed God, which was committed to my trust.” (1 Timothy 1:9–11) (KJV 1900)
Why did God remove the Old Covenant (Physical Israel) replacing it with the New Covenant?
““Indeed, a time is coming,” says the Lord, “when I will make a new covenant with the people of Israel and Judah.It will not be like the old covenant that I made with their ancestors when I delivered them from Egypt. For they violated that covenant, even though I was like a faithful husband to them,” says the Lord.“But I will make a new covenant with the whole nation of Israel after I plant them back in the land,” says the Lord. “I will put my law within them and write it on their hearts and minds. I will be their God and they will be my people.” (Jeremiah 31:31–33) (NET)
“When he speaks of a new covenant, he makes the first obsolete. Now what is growing obsolete and aging is about to disappear.” (Hebrews 8:13) (NET)