Why was Abel accepted by God but Cain was not?
God had asked for a blood sacrifice and promised acceptance to those who obeyed. Abel was accepted because he did exactly what God told him to do. Cain was not accepted because he rejected that instruction, rationalized it, and chose to go his own way rather than obey.
Les explains that God had offered restoration and reconciliation if they would just do what He asked them to do—bring a blood sacrifice—and He would accept them. Abel was accepted because he did what God told him to do. Cain rejected it, rationalized it, and thought he would go his own way. The contrast shows that acceptance rested on obedience to God's prescribed blood sacrifice, while Cain's rejection and self-directed reasoning kept him outside that acceptance.