Because they lacked faith, the law Abraham walked by.
It is a restoration to the land. The oath of God spoke to his faithfulness. God keeps his oath.....
Unto thy seed...... The law 400 years later cannot disannul God's oath.
Gen 15:18 In the same day the LORD made a covenant with Abram, saying, Unto thy seed have I given this land, from the river of Egypt unto the great river, the river Euphrates:
Ex 33:1 And the LORD said unto Moses, Depart, and go up hence, thou and the people which thou hast brought up out of the land of Egypt, unto the land which I sware unto Abraham, to Isaac, and to Jacob, saying, Unto thy seed will I give it:
Le 26:42 Then will I remember my covenant with Jacob, and also my covenant with Isaac, and also my covenant with Abraham will I remember; and I will remember the land.
De 9:5 Not for thy righteousness, or for the uprightness of thine heart, dost thou go to possess their land: but for the wickedness of these nations the LORD thy God doth drive them out from before thee, and that he may perform the word which the LORD sware unto thy fathers, Abraham, Isaac, and Jacob.
De 9:27 Remember thy servants, Abraham, Isaac, and Jacob; look not unto the stubbornness of this people, nor to their wickedness, nor to their sin:
De 29:13 That he may establish thee to day for a people unto himself, and that he may be unto thee a God, as he hath said unto thee, and as he hath sworn unto thy fathers, to Abraham, to Isaac, and to Jacob.
De 30:20 That thou mayest love the LORD thy God, and that thou mayest obey his voice, and that thou mayest cleave unto him: for he is thy life, and the length of thy days: that thou mayest dwell in the land which the LORD sware unto thy fathers, to Abraham, to Isaac, and to Jacob, to give them.
De 34:4 And the LORD said unto him, This is the land which I sware unto Abraham, unto Isaac, and unto Jacob, saying, I will give it unto thy seed: I have caused thee to see it with thine eyes, but thou shalt not go over thither.
The passover is a memorial sacrifice, and a feast to memorialize the faithfulness of God to his oath concerning them.
De 7:8 But because the LORD loved you, and because he would keep the oath which he had sworn unto your fathers, hath the LORD brought you out with a mighty hand, and redeemed you out of the house of bondmen, from the hand of Pharaoh king of Egypt.