Some denominations include that book. And even in the more restrictive bibles, there are a few references to the books of Enoch to let us know that it was considered perhaps just as important as the rest of the scriptures.

There were many fragments with the more intact Dead Sea Scrolls, but both the Torah and Bible use only their own selections of the Ancient Hebrews' books, and in different orders.

Presumably the selections were to suit whoever was in theological and political power at the time; with their own reasons for using or rejecting each work.

There might have been others long since lost totally. Whoever are the "they" saying it, the lack of religious literature from that 4 centuries BCE may be by a combination of reasons. The scribes may have not needed to write new ones anyway as their beliefs had become entrenched. They may have written some now long-lost accidentally. It's even plausible some writings were rejected, perhaps even destroyed, for dogmatic reasons, with no record of this happening.

Probably because the Christian "OT" follows cannon of the Jewish Tanach which was already considered closed by the time Enoch was written. I suppose the church could have included it with the books of the Apocrypha, but then the Protestants rejected those books by the late 1800's anyway so you'd be back to a bible that didn't include Enoch.