Why isn't the Book of Enoch in the Bible?

The Book of Enoch is absent from most Bibles because it was left out when the Jewish and Western Christian canons were fixed, even though it was widely read in Second Temple Judaism and early Christianity. The New Testament Epistle of Jude quotes it directly, and it was known to the writers of 2 Peter and Revelation, yet it formed no part of the Hebrew Bible, and the rabbis along with most church fathers declined to receive it as scripture. This stemmed partly from its late, pseudonymous authorship - composed centuries after the historical Enoch - and from its expansive angelology (Canon & Reception - Canon and reception of the Book of Enoch).

It is not a banned or lost work so much as one tradition’s scripture and another’s apocryphon. The complete text survived only in Ge’ez because the Ethiopian Orthodox Tewahedo Church has always kept 1 Enoch, together with Jubilees, as canonical scripture. In that church’s broad canon of 81 books, Henok is simply part of the Bible and has been read and revered for well over a millennium, never treated as a curiosity or appendix (Ethiopian Canon - 1 Enoch in the Ethiopian Canon; Ethiopian Canon - 1 Enoch in the Ethiopian Canon: Scripture, not apocrypha).