Glory to Jesus Christ!
I will try to be brief, but that means I will leave a great deal unsaid.
It helps to know a little history of the Old Testament scriptures.
The Old Testament was written over a 1500 year period (approximately) and in that time Jewish (and Israelite) understanding of God and His ways matured. The more conservative of the Jewish synagogs were reluctant to accept later writings. This process continued throughout the entire epoch. There was no firm Canon of scripture in the Jewish mind then, each synagog had it’s own idea what a “had-to-have” set of books would be.
Jews had been migrating out of the Judean region for hundreds of years. After the time of Alexander the Great the whole eastern half of the Mediterranean world had Greek speaking rulers and upper classes. Those Jews began to assimilate into the culture and eventually adopted Greek as their first language. Similarly Aramaic became the common tongue in Palestine very early on and Hebrew became a liturgical language (in a sense a “dead” language), not spoken in the markets or the homes of ordinary Jews.
A translation to Greek of the extant Hebrew scriptures was made in Alexandria in about 282BC - more than 300 years before the public ministry of Christ. This translation became a popular addition to the collections of synagogs all over the ancient world, including in Palestine itself where Helenization was proceeding apace under the Seleucids and later the Romans. This collection of translations became a Canon of sorts by default.
It is the collection of books included in the Septuagint that the early Christians (Catholics/Orthodox) used. The order you will find them “tucked in” is the order the Christian church has used them from a very early period.
So why are the Deutrocanonicals sometimes isolated in a section of their own behind the other Old Testament books?
After the disastrous rebellion of the Jews in Palestine it seemed likely that Judaism would dissolve into the mists of history without a Temple cult and a legal standing. Finally a first-ever council of Pharisaic Rabbis was held at Yavneh, there was an academy of scholars already there and they addressed the issue of how to restore Judaism and protect it from the influence of those darn Christians who seemed to be spiriting away far too many Jewish people.
Part of their answer was to do a little pruning of the books acceptable in a synagogs collection. Some of the writings of a later date were suspect, and it was determined that no books in the Septuagint should be used if that has no Hebrew version to authenticate it. The fear was that these writings may have been first written in Greek and could be considered spurious and misleading.
What we see in the DeuteroCanonicals did not make the Rabbi’s recommended reading list.
This ruling did not affect the Catholic/Orthodox churches because they first probably were unaware of the proceedings and secondly would have not recognized this group of non-Messianic Jews to have any influence over them. The Christians continued to use the Septuagint intact. In later years the difference in Canons did get attention from the Church Fathers, Jerome especially, but they decided that they should stick with what they knew.
Many Protestant versions of the Bible included the DeuteroCanoniucals as a seperate section. My KJV is like that too. The NIV has apparently never even bothered to translate the Deuterocanonicals, apparently they would decide that issue for us.
Catholics and Orthodox will have all of the elements in proper place, not “tucked in” as such but actually where they have always normally been. No flipping pages in the Book of Daniel or Esther.
