Proteus wrote:
Why was the Book of Enoch not included in TaNaKh?
Proteus, There are at least two books of Enoch, and possibly a third one as well, although it is Enoch I that is commonly referred to as the Book of Enoch.

Regarding your question, I was unable to find a hard-and-fast answer to it.  According the the Hebrew Wikipedia page on the apocryphal books, there is no agreement among researches regarding the reasons for the exclusion of those books from the Jewish canon.  Even though common opinion in the past claimed it was for theological reasons, this opinion appears less and less likely for various reasons.  The available evidence indicates that the exclusion of the apocryphal books was more likely due to historical circumstances based on chronology rather than theological ones, since most of those books were written in a much later period than the books in the Hebrew Bible.  However, that cannot be the singular and deciding reason since, in the opinion of the researchers, the Book of Daniel (which is included in the canon) post-dates the Book of Enoch by about 100 years.

UriYosef

Our raison d'être:

WHOSOEVER DESTROYS A SINGLE SOUL OF ISRAEL, SCRIPTURE IMPUTES [GUILT] TO HIM AS THOUGH HE HAD DESTROYED A COMPLETE WORLD; AND WHOSOEVER PRESERVES A SINGLE SOUL OF ISRAEL, SCRIPTURE ASCRIBES [MERIT] TO HIM AS THOUGH HE HAD PRESERVED A COMPLETE WORLD. (Babylonian Talmud, Tractate Sanhedrin, 37a)

The fruits of our effort:

The battle against spiritual terrorism is being won, one soul at a time!!!