Why didn’t God send Yeshua to redeem humanity right after Adam sinned instead of giving the Law?

The answer is simple: It was the choice God made. Obviously, God had multiple options as to how He could have handled both the angelic and human sin, and He chose the one He put into effect and waited for the “fullness of time” before sending His Son (Galatians 4:1-7). He did so without telling us why. Hence, there is no other answer we can give except what God has chosen to reveal to us in His Word.

Now I say, as long as the heir is a child, he does not differ at all from a slave although he is owner of everything,² but he is under guardians and managers until the date set by the father.³ So also we, while we were children, were held in bondage under the elemental things of the world.⁴ But when the fullness of the time came, God sent forth His Son, born of a woman, born under the Law,⁵ so that He might redeem those who were under the Law, that we might receive the adoption as sons.⁶ Because you are sons, God has sent forth the Spirit of His Son into our hearts, crying, “Abba! Father!”⁷ Therefore you are no longer a slave, but a son; and if a son, then an heir through God.(NASB)