Why isn't Cain in Adams genealogy?
Genesis 4:25 And Adam knew his wife again, and she bore a son and named him Seth, "For God has appointed another seed for me instead of Abel, whom Cain killed."
Genesis 5:3 And Adam lived one hundred and thirty years, and begot a son in his own likeness, after his image, and named him Seth. 4 After he begot Seth, the days of Adam were eight hundred years; and he had sons and daughters.
We see from this that Adam fathered many sons in the 800 years after Seth was born, but none of them are named or listed in Adam’s genealogy.
Seth’s genealogy continues to Noah, at which time the line of Cain and Adam’s other sons came to an end in the flood.
Cain’s genealogy and those of Adam’s other sons serve no purpose in God’s plan of salvation so there would be no point them being given.
I hope you find this helpful.