Why inherit the first sin of Adam, and not his subsequent sins?

The Catholic Answers

Only in the matter with which the first sin was connected was Adam constituted the supernatural head of the race. After his sin as head of supernaturally elevated human nature, he sinned as a private individual and independently of God’s universal decree for the human race as such.

Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty