According to Reformed Theology, why did the 70 Jewish translators include the Apocrypha?

It is true that the Jews ultimately did not accept the apocryphal books you listed in their canon, in a gradual process over several centuries at least spanning 1st century BC and 1st century AD. There was no definitive answer, but a lot of pointers showing the development, shown in the BIBLE CANON article of the 1906 Jewish Encyclopedia. Especially after the 1st century AD, in opposition to Christian usage of the Greek Septuagint to prove the truths of Christianity (such as how Septuagint translated Hebrew word almah into virgin in Isa 7:14 which the Christians used as one proof of Jesus's divinity), the Rabbinic Jews (whose precursor was the Pharisees) preferred the older Hebrew of the Old Testament, thus abandoning the use of Septuagint.

The early Christians who became more and more gentile in demographics and who understood only Greek (not Hebrew), understandably adopted the Septuagint more and more because they needed the prophetic background as a proof of their "New Testament" beliefs, so in ANOTHER gradual process completely separated from the Rabbinic Jews, the apocryphal books increasingly became part of their canon. But as this excellent article shows, the acceptance of the apocryphal books were not unanimous among the Church Fathers.

Starting with the Reformation, as a reaction to some abuses in the Catholic church making use of the apocryphal books to support the doctrines which the Reformers opposed (indulgences, purgatory, prayers to saints, etc.), as well as the earlier influence of Renaissance Humanism's ad fontes movement, Protestant communities started to create new editions of the whole Bible from the original Greek and Hebrew sources instead the Latin of the Vulgate, translated into their national languages such as English and German. Critical self reflection of the canon, coupled with the Protestant movement ethos to go back to the original theology of Christianity, thus formed the background on why the apocryphal books were rejected by the reformers.

The short Introduction to the Old Testament essay by the renowned Bible scholar Bruce M Metzger included in the 1962 Oxford Annotated Bible (RSV) is quite valuable to give us a contextual narrative of how the OT books were edited and became Scripture over several hundred-year period and how the apocryphal books were treated differently. The Preface to the Revised Standard Version is also very helpful to serve as a short history of the Bible in the English language (started with Tyndale's project in 1535). Archive.org has a clean pdf of all the essays plus the 39 OT books (without the apocrypha).

As you can see from the articles I referenced above, the list of canonical books varies according to various criteria: holiness (inspired by God), what counted as "Scriptures", what are edifying, what can be used as a source of theology, etc. Consider also the different communities, whose identity development over centuries played into the making of the list: those who produced the Septuagint, Early Christians, Rabbinic Jews, Catholics, Protestants, etc. each understandably had varying interpretation of what the apocryphal books meant for their communities.

So by 1561, mere decades from Martin Luther's famous 95 theses, the Reformed Theology "fathers" already enshrined the "new canon" excluding the apocrypha in their Belgic Confession Articles 4-6 with the following reasons:

ARTICLE 4: We include in the Holy Scripture the two volumes of the Old and New Testaments. They are canonical books with which there can be no quarrel at all. In the church of God the list is as follows:

In the Old Testament, the five books of Moses— Genesis, Exodus, Leviticus, Numbers, Deuteronomy; the books of Joshua, Judges, and Ruth; the two books of Samuel, and two of Kings; the two books of Chronicles, called Paralipomenon; the first book of Ezra; Nehemiah, Esther, Job; the Psalms of David; the three books of Solomon— Proverbs, Ecclesiastes, and the Song; the four major prophets— Isaiah, Jeremiah^a, Ezekiel, Daniel; and then the other twelve minor prophets— Hosea, Joel, Amos, Obadiah, Jonah, Micah, Nahum, Habakkuk, Zephaniah, Haggai, Zechariah, Malachi.

In the New Testament, the four gospels— Matthew, Mark, Luke, and John; the Acts of the Apostles; the fourteen letters of Paul— to the Romans; the two letters to the Corinthians; to the Galatians, Ephesians, Philippians, and Colossians; the two letters to the Thessalonians; the two letters to Timothy; to Titus, Philemon, and to the Hebrews; the seven letters of the other apostles— one of James; two of Peter; three of John; one of Jude; and the Revelation of the apostle John.

^a “Jeremiah” here includes the Book of Lamentations as well as the Book of Jeremiah.

ARTICLE 5: We receive all these books and these only as holy and canonical, for the regulating, founding, and establishing of our faith. And we believe without a doubt all things contained in them— not so much because the church receives and approves them as such but above all because the Holy Spirit testifies in our hearts that they are from God, and also because they prove themselves to be from God. For even the blind themselves are able to see that the things predicted in them do happen.

ARTICLE 6: The Difference Between Canonical and Apocryphal Books We distinguish between these holy books and the apocryphal ones, which are the third and fourth books of Esdras; the books of Tobit, Judith, Wisdom, Jesus Sirach, Baruch; what was added to the Story of Esther; the Song of the Three Children in the Furnace; the Story of Susannah; the Story of Bel and the Dragon; the Prayer of Manasseh; and the two books of Maccabees.

The church may certainly read these books and learn from them as far as they agree with the canonical books. But they do not have such power and virtue that one could confirm from their testimony any point of faith or of the Christian religion. Much less can they detract from the authority of the other holy books.