I can forgive your conflating the Watchers with the Nephilim, as that confusion is historically well-attested, and Bob and Dale got the basics right (and Reza Aslan didn't say anything too absurd). But I can't McGowan's supposition that 1 Enoch was left out of the canon because it portrayed God as too merciful stand.
First of all, if you read all five books of 1 Enoch, the punishment of the wicked is a pretty thoroughgoing theme. Sure, there is absolutely a strong strand of divine mercy in all apocalyptic literature; the whole idea is that the wickedness of this world is the result of a disruption of the divine order that will be restored in the eschatological future. Apocalypticism is a message of hope. Which is exactly why the notion that Enoch was omitted from the canon for being too hopeful is garbage.
Scholars have suggested a number of factors that led to Enoch's rejection from most Christian biblical canons:¹
- The most important factor was perhaps a recognition that Enoch was rejected by the emerging Jewish canon, as part of a more general trend of marginalizing apocalyptic and mystical speculation by the nascent Rabbinic tradition.
- Its authenticity was doubted. Some Christians questioned whether the book was really written by Enoch, or how a book written before the flood could have survived at all.
- While 1 Enoch was accepted as an authoritative source of historical and cosmological information by many early Christian fathers, there isn't much indication that it played an important role in the liturgy (except perhaps in Egypt and Ethiopia). Books that weren't read in churches were less likely to be canonized.
- After the third century, proto-orthodox Christianity was less and less comfortable with apocalyptic speculations. Expectations of an imminent end of the world were fading, and eschatologically-oriented works (like the Book of Revelation) were either marginalized or allegorized by the proto-orthodoxy.
- At the same time, Enochic traditions were being embraced by groups outside the proto-orthodoxy, like the Manichaeans, the Montanists, and various "gnostic" groups. This made them more suspect to those in the emerging "mainstream."
- The story of the fallen Watchers bringing evil to earth was not compatible with the increasingly dominant idea of human sin coming from Adam's disobedience in Eden. It was easier to reinterpret the "sons of God" in Genesis 6 as the offspring of Seth corrupted by the daughters of Cain than it was to incorporate the Watchers story into the what became the doctrine of Original Sin.
For my money, it is far more important to realize that Enoch was considered authoritative, even inspired, by many early Christians, because it demonstrates that the books that eventually formed the canon were only a fraction of the sources that were important to the early church. While the bombastic narrator of Bible Secrets Revealed seems intent on painting the formation of the canon as the suppression of "secret" or "forbidden" ideas, it was a far more complex and interesting process than that.