Jude Cites Non-Canonical Texts

Here is the detail that settles the question. Go back to Genesis 6 and look for what happened to the sons of God afterwards. It is not there. Genesis records the transgression and then moves to the flood. It "records no fate for the bene elohim."
But Jude 6 knows their fate precisely - kept in eternal chains, under gloomy darkness, until the judgment of the great day. That specific tradition comes from 1 Enoch 10 and 21. Jude did not get it from Genesis, because Genesis does not supply it.
And he states it without a word of explanation, which tells you what he assumed about his readers.
Scripture links:
- Jude 6 - "he has kept in eternal chains under gloomy darkness until the judgment of the great day."
- Genesis 6:1-4 - The transgression, with no stated consequence for the sons of God.
Jude fills a gap Genesis leaves open, from a book his readers already knew.