As we read, God was grieved that he created us, and all living things. It seems the descendants of Cain and others caused great sin, violence and horrible things.
I go back to Adam, who failed to protect and help his own wife obey God in the garden, so Adam failed to instruct Cain in how to offer sacrifice to God. Both meat and grain were good offerings, but Abel had a correct and humble spirit.
Adam failed to teach Cain. I find it amazing that God still tried to walk among the descendants of Adam and Eve until he could not tolerate it anymore.