It seems to me that your question (of curiosity) really lies with what should be the criteria for deciding if a book is inspired or is not inspired (ie. Belongs in the Bible or doesn't). And this is a very important question to ask. It leads into the question of who is to say what the criteria is. Even if a book of the NT has a quotation in it that says "thus saith the Lord" refering to an OT book, we still would have to be sure of the particular NT books status as inspirational that had the quote in it. Even the Koran (and numerous other writings outside the Bible, whether Catholic, or Protestant) call themselves inspirational. We all agree that this alone cannot suffice to decide on a book's canonical status. So what are the "rules" for canonicity (I think that's the right spelling)? And who has the authority to come up with them?
We must remember that Christ, while he was on earth, did not just sit down, write the NT in it's entirety, hand the Bible (OT and NT) over to the Apostles, then told them to go out, make copies, and hand the Bible out to all the world. He instituted the Church to infallably teach the Word of God to the entire world, through preaching. The Bible was written during the time of the Apostles, but no official canon was in place to say what was a part of the Bible and what wasn't.
It wasn't until 350-400 years after Christ that there was a canon in place (unless you want to make the argument that it wasn't until the Council of Trent in the 1500's that the Catholic Bible came to be, and of course the Protestant Bible didn't come into it's own official canonical status until the 1500's). This means that for 350-400 years (or 1500+ years if going with the Council of Trent/Protestant Reformation argument) the Church and Holy Tradition were the only known sources of Divine Teaching. Sacred Scripture was of course used, but there were many false letters circulating around, the authors claiming to be Paul, when in fact they were not. So the laity had to turn to the Church to know what writings were true and which weren't, and also what books were inspired and which werent.
And if the Protestant Bible was the "right" Bible then why did God wait 1500+ years to give it to Christians, especially if the notion of Sola Scriptura is correct? Were the original Christians wrong for listening to the authority of the Church? And were their beliefs wrong as well, since they line up with the Catholic teaching of today? We must remember that God instituted the Church and the Church (with God's guidance) wrote the NT and came up with the entire Bible, the Bible did not come up with the Church. Without a divinely appointed infallable authority outside of the Bible to come up with the Bible in the first place we have no Bible to turn to. So who is this divinely appointed infallable authority, it is the Catholic Church.
Every Protestant I've spoken to, from Pastors and Ministers to lay people, all admit to their own fallability and their lack of authority. They all say the Bible and the Bible alone is the sole rule of faith. They dont say the Bible and the Protestant Reformers, or the Bible and the Jews who came up with their own OT canon in circa 100AD (which some Protestants like to use to support the Protestant OT canon since it is, I believe, the same as this Jewish canon), just the Bible. But the Bible didn't come up with itself, and could never come up with itself. It didn't drop from the heavens (in it's official status as a complete written book) on the lap of either the Catholic Church or the Protestant Church. It came from God's inspiration, through the Holy Spirit, through men in the Church, through the decisions made by the infallability of the Catholic Church (which was guided by God, particularly the Holy Spirit, in making this canonical decision). Protestants either have to say they believe in their own infallability (Sacred Tradition and/or Protestant Magesterium) or say that everyone has just as much a right to come up with their own Bible as anyone else does (including the Protestant Reformers). Neither of these things they will say though, because they don't believe in either of these ideas. So what are we left with? In my opinion it's either the Catholic Church and It's Bible or no Bible at all. I know this sounds harsh, but I really don't see any other answer. The Church is the infallable authority set up by our Lord Jesus Christ to spread and teach the Good News. So the Church also needed to infallably know what books, or writings, were in fact inspired by God and which weren't. Anyone can come up with their own checklist for which books belong in the Bible and which don't but who is to say? Catholics hold that the Church was set up to decide. The Protestants say that the Bible itself is the sole rule of faith. This notion contradicts itself, in my opinion, and that's one reason why I turn to the Catholic Church for infallable teaching. There is much more that can be said for "proving" that the Catholic Church is "the" authority, but I just wanted to try and concentrate on the discussion at hand.
I am interested in hearing everyone's responses to these issues and comments and as always, welcome a friendly and lively debate from all. Thank you, in advance, for allowing me to voice my beliefs and for reading this post.