A lot had happened since God created the world and said, “It is good.”
Adam and Eve had been given their mandate from God, including the one “thou-shalt-not.” God had instituted marriage. Adam and Eve had chosen to rebel against God and His command. Fear and blameshifting had entered the picture along with the sin of rebellion. And finally, God had pronounced the curse resulting from sin, along with a hint at His plan of salvation.
In Genesis 4 we get a snapshot of life outside the Garden and a firsthand look at the downward spiral to which Adam and Eve had opened the door. (You can read more of this post at A Look at the Book.)