Why did Abraham give all that he had to Isaac?
Abraham gave all that he had to Isaac because Isaac was the promised son of the covenant. In Genesis 17, God promised Abraham that Sarah would bear a son, that she would be a mother of nations, and that kings of people would come from her. Isaac was therefore the son through whom those covenant promises would continue.
Les explains that verse 5 must be understood by comparing Scripture with Scripture, so he takes us back to Genesis 17. There God changed Abram and Sarai’s names to Abraham and Sarah, with the letter H associated with Yahweh or Jehovah, as He brought them to the place of the covenant promises. God told Abraham that He would bless Sarah and give him a son by her, and promised that Sarah would be a mother of nations and that kings of people would come from her. Even though the promise had not yet happened—Abraham had waited perhaps fifty years, from his first call until Isaac was born at a hundred—Isaac was the promised son of that covenant. Thus Abraham gave all that he had to Isaac because Isaac was the son of promise through whom the covenant promises would continue.
And God said unto Abraham, As for Sarai thy wife, thou shalt not call her name Sarai, but Sarah shall her name be.
And I will bless her, and give thee a son also of her: yea, I will bless her, and she shall be a mother of nations; kings of people shall be of her.