Why was Abram given no part in making the covenant in Genesis 15?
God arranged it so the covenant would be from God and God alone. Abram was given no part to make sure he had nothing to do with the covenant's responsibility, showing it rested wholly on God's promise rather than Abram's actions.
Les explains that in Genesis 15 God is dealing with Abram, who has already received the covenant and its promises but asks for proof that he will inherit the land. The next verses describe the then-typical ancient Babylonian means of transferring real estate by blood covenant: animals were killed, the carcasses parted, and a walkway left between the halves. Les teaches that God comes down to do the very same thing, but in order for this covenant to be as all covenants are from God and God alone, God makes sure Abram has nothing to do with the covenant responsibility. The context then asks what God does with Abram, underscoring that Abram is removed from the covenant-making transaction so it is solely God's doing.
And when the fowls came down upon the carcases, Abram drove them away.
And when the sun was going down, a deep sleep fell upon Abram; and, lo, an horror of great darkness fell upon him.