Greetings in that Divine and Most Precious Name of Our Lord and Savior Jesus Christ!
Their excuse were the books were not in Hebrew, and therefore should not be included.
Of course, many books included (such as Daniel) are not entirely in Hebrew...and some of the books rejected by the Masorites were originally in Aramiac and/or Hebrew (as confirmed by the Dead Sea Scrolls and other archeological finds).
Protestants just seem unsure about all of this, being confronted with a period of history usually completely ignored by their backgrounds. They raise the Bible up as the only measure of faith and completely reject Tradition (and therefore the proper biblical hermaneutic and history), but are oddly ignorant as to how the Bible was formed by the Church and how there still isn't complete uniformity. Many I speak with are surprised to know that Scripture sometimes authoritatively quote books not included in the canon (St. Jude's Epistle quoting the Book of Enoch, for example).
Yes, this is the fundamental paradox of sola scripture faith (like I was raised on) and the Sacramentality of the Orthodox/Catholic which teaches the Divine Mysteries. In the Protestant tradition (aside from Anglicans and Lutherans), all the Sacraments are believed as symbols, and those who believe in the Mysteries are spurned as overly superstitious and in the Apostle Paul's terms, "old womanly." However, this is just absolutely silly. Sacramentality is the cornerstone of the Church, that God directly and powerfully operates in the world, in our realm, through the Sacraments/Divine Mysteries. Baptism is not a symbol of joining the death and resurrection of Christ, it is literal and through the Holy Spirit makes all things new. Reconciliation is not a symbol of absolution and forgiveness, it is literal, as the Holy Spirit cleanses the heart of the weights of sin which bring us down in our lives and inevitably kill us. The Offering is not a symbol or mere commemoration of Christ's Passion, it is literal, it is the Real Presence of our Lord and Savior in the Flesh and Blood, and it heals us of our wounds, it gives us the new life of Christ, it joins us into His Body and makes us One. Priests are not symbolic messengers of God, they are the very Apostles, through the Holy Spirit and the laying of hands according to the Mystery of Ordination. Our weddings are not symbols of unified relationships, but rather through the Holy Spirit the two flesh are joined and made one, and to God these are never separated.
However from the Protestant tradition all of this is a superstition, a myth, a fable. They are gravely mistaken as they willfully (and also woefully) deny that very saving, operative Grace which God gives us in the Mysteries. Thus they rely on the only Mystery they have left, the Scriptures. However even our Lord said, "Search the Scriptures for in them you are supposing to find eternal life, but will you not just turn to Me who these speak of?"
In reality, Protestants are actually more superstitious about the Bible than they mistake us to be about the Sacraments. This is why they reject anything essentially but their own interpretation of what is the canon of the King James Version. Many suggest the NIV or even the NKJV are as its been suggested above, "diabolical." Imagine that! What a lack of faith, don't they believe that God can speak for Himself directly in any translation, in any version of the Scriptures? Even HIM Haile Selassie told us that, "What ever language the Scriptures have been translated into, they remain the very word of God."
Further, they are in literal ignorance of the historical inaccuracies of their limited perspective. They accept the Hebrew bibles over the Septuagint even though this is historically inaccurate. The Hebrew Bible is relatively young in comparison to the Greek Septuagint, the oldest complete copies we can find are perhaps no older than a thousand years. Whereas the Septuagint has been found complete copies much older. We in the Church accept the Septuagint as a legal Canon and translation because as its been pointed out above, it was the Scriptures of Our Lord and of the Apostles. It is thee authoratative version, and yet we are not legalistically caught on this. In the Orthodox the Canon of the Scriptures varies. In the Ethiopian Orthodox Tewahedo Church a unified, national canon wasn't established until the Haile Selassive Version of the Amharic/Ge'ez Bible in the 1930s.
Even more paradoxically, they openly defy and reject books quoted in the KJV such as Enoch, Jubilees, Book of Gad the seer, the Books of King Solomon, and many others (Enoch and Jubilees currently still exist ONLY in the Ethiopian Orthodox canon by the way
) which seems to make no sense. These believe the Bible to be absolutely infallible, to be as perfect as God Himself, and yet they reject their own supposedly perfect book? I honestly don't get it, never did, and that is precisely why I converted to Orthodox in the first place, because my entire life I could never relate to those around me in the Protestant traditions who lived this kind of pseudo-Christianity, one which lacked reason, which lacked substance. I adore the sincere faith of many Protestants, I am not criticizing the people, rather their tradition (IE, the structure of their Church/Worship, not the worshipers) which is hopelessly confusing.
Much like others have mentioned, the canon of the Scriptures varies from region to region, jurisdiction to jurisdiction, but we do not demonize each other over this as some of the Protestants do, but that is because of the fundamental difference between our worship. All the Protestant tradition offers and relies upon are their versions of the Bible, which they take literal in every sense. We in Orthodox have the Divine Mysteries to sustain, to teach, to uplift, to heal, to console, to give life to us.
The Sola Scripture mentality is a depressing one because it puts limits on God, it limits God to whatever is in the Book, as if God could somehow be limited?