1 Then again Abraham took a wife, and her name was Keturah. 2 And she bare him Zimran, and Jokshan, and Medan, and Midian, and Ishbak, and Shuah. 5 And Abraham gave all that he had unto Isaac. 6 But unto the sons of the concubines, which Abraham had, Abraham gave gifts, and sent them away from Isaac his son, while he yet lived, eastward, unto the east country.
After Sarah’s passing Abraham took another wife, who bare him six sons that we hardly hear anything about. Abraham gave gifts to all of these sons and then sent them away to establish their own households, retaining the full inheritance for Isaac. Giving the inheritance to a single son was a common custom of the time, but more than that, God had chosen Isaac to carry on His covenant, and not the other sons.
Similarly, God would select only one of Isaac’s sons to receive the covenant promise: Jacob. But then, in the next generation, all of Jacob’s twelve sons would be included in the covenant, not just one. Thus, the decision for how the covenant was passed along to each generation was God’s to make, and then the father’s inheritance followed suit.
While God’s reasoning for which sons he chose for the covenant are not always elaborated, we do see in the case of Jacob and Esau that God selected the son who was more faithful. It seems likely that the continuation of the covenant was based on one’s worthiness, rather than a matter of random election.