The biblical book of Genesis has a strong chronological backbone, going up to Abraham (the book's first major character, the other two being Jacob and Joseph), upon which further stories (such as the two Creation Accounts, the Fall of Man, Cain and Abel, the Tower of Babel, the Fall of Angels, Noah's Flood, etc.) are added, as muscles unto a sustaining skeleton. These inserted episodes, however, do not meaningfully affect the flow of the overarching narrative upon which they are grafted; allow me to demonstrate:
Genesis 1:1 ¶In the beginning God created the heaven and the earth. 1:26 ¶And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth. 1:27 So God created man in his own image, in the image of God created he him; male and female created he them. 1:28 And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth upon the earth. 5:3 And Adam [Man] lived an hundred and thirty years, and begat a son in his own likeness, after his image; and called his name Seth: 5:5 And all the days that Adam lived were nine hundred and thirty years: and he died. 5:6 ¶And Seth lived an hundred and five years, and begat Enos: 5:8 And all the days of Seth were nine hundred and twelve years: and he died. 5:9 And Enos lived ninety years, and begat Cainan: 5:11 And all the days of Enos were nine hundred and five years: and he died. 5:12 And Cainan lived seventy years, and begat Mahalaleel: 5:14 And all the days of Cainan were nine hundred and ten years: and he died. 5:15 And Mahalaleel lived sixty and five years, and begat Jared: 5:17 And all the days of Mahalaleel were eight hundred ninety and five years: and he died. 5:18 And Jared lived an hundred sixty and two years, and he begat Enoch: 5:20 And all the days of Jared were nine hundred sixty and two years: and he died. 5:21 ¶And Enoch lived sixty and five years, and begat Methuselah: 5:23 And all the days of Enoch were three hundred sixty and five years: 5:24 And Enoch walked with God: and he was not; for God took him. 5:25 ¶And Methuselah lived an hundred eighty and seven years, and begat Lamech: 5:27 And all the days of Methuselah were nine hundred sixty and nine years: and he died. 5:28 ¶And Lamech lived an hundred eighty and two years, and begat a son: 5:29 And he called his name Noah, saying, This same shall comfort us concerning our work and toil of our hands, because of the ground which the LORD hath cursed. 5:31 And all the days of Lamech were seven hundred seventy and seven years: and he died. 5:32 And Noah was five hundred years old: and Noah begat Shem, Ham, and Japheth. 7:6 And Noah was six hundred years old when the flood of waters was upon the earth. 9:29 And all the days of Noah were nine hundred and fifty years: and he died. 11:10 ¶Shem was an hundred years old, and begat Arphaxad two years after the flood: 11:11 And Shem lived after he begat Arphaxad five hundred years. 11:12 And Arphaxad lived five and thirty years, and begat Salah: 11:13 And Arphaxad lived after he begat Salah four hundred and three years. 11:14 And Salah lived thirty years, and begat Eber: 11:15 And Salah lived after he begat Eber four hundred and three years. 11:16 And Eber lived four and thirty years, and begat Peleg: 11:17 And Eber lived after he begat Peleg four hundred and thirty years. 11:18 And Peleg lived thirty years, and begat Reu: 11:19 And Peleg lived after he begat Reu two hundred and nine years. 11:20 And Reu lived two and thirty years, and begat Serug: 11:21 And Reu lived after he begat Serug two hundred and seven years. 11:22 And Serug lived thirty years, and begat Nahor: 11:23 And Serug lived after he begat Nahor two hundred years. 11:24 And Nahor lived nine and twenty years, and begat Terah: 11:25 And Nahor lived after he begat Terah an hundred and nineteen years. 11:26 And Terah lived seventy years, and begat Abram, Nahor, and Haran. 11:27 ¶And Haran begat Lot. 11:28 And Haran died before his father Terah in the land of his nativity, in Ur of the Chaldees. 11:29 And Abram and Nahor took them wives: the name of Abram's wife was Sarai; and the name of Nahor's wife, Milcah, the daughter of Haran, the father of Milcah, and the father of Iscah. 11:30 But Sarai was barren; she had no child. 11:31 And Terah took Abram his son, and Lot the son of Haran his son's son, and Sarai his daughter in law, his son Abram's wife; and they went forth with them from Ur of the Chaldees, to go into the land of Canaan; and they came unto Haran, and dwelt there. 11:32 And the days of Terah were two hundred and five years: and Terah died in Haran.
Notice, then, that the explicit mention of Seth being created in man's (Adam's) image plays a vital role in tying together (the beginning of) this major underlying historical thread, whereas such a mention in the case of the interwoven Cain and Abel interlude would have proven completely unnecessary or superfluous.