@David_Jay, it looks like your reply was swallowed up by formatting issues, so I'll (somewhat ironically) repeat it. :P Always good to hear from you on this!
"Canonicity" was a response to the challenge of Marcionism, and understanding a bit about that explains why certain books did not end up in the Christian canon.
Marcion of Sinope was a bishop who, influenced by Gnosticism, became a heretic. Like the Gnostics, Marcion taught that salvation was possible only through gaining special "gnosis," or knowledge. Holy writ (religious texts) from various cults were often believed to have the ultimate form of revelation hidden in their words, but they could only be understood by those "chosen" to understand them.
Marcion adapted Gnostic belief into his form of Christianity. He rejected the God of Abraham in favor of Jesus (in the primitive Church it has already been decided that Jesus was some sort of epiphany of the God of Abraham, an "incarnation" of the Shekinah or "Light" of God [see John 1:4-9]). Marcion claimed that Jesus was not the YHVH but a superior and kinder god, and that Paul was his primary apostle. Marcion also taught that though only "chosen ones" had the ability to understand the "hidden knowledge" found in holy writ, this "gnosis" could be disseminated by the chosen to those who joined them.
As for "holy writ" Marcion made a "rule" (in Greek, KANON) that only select letters of the apostle Paul were true, and the gospel of Luke (with all Jewish references edited out). Marcionism was born and attracted many followers.
In response, the Church (which up till that time believed that the Hebrew Scriptures were the only form of holy writ inspired of God) responded by excommunicating Marcion. But the genie had been let out of the bottle, so to speak, never to get back in. Were the books of Luke and some of Paul’s letters inspired like the Hebrew Scriptures?
Marcion got his idea about these books because early Christian worship services were liturgical, actually imitations of Jewish synagogue services which read the Torah and the other holy books on a schedule much like Jews do today (and Christians that follow the Revised Common Lectionary based on the Roman Catholic liturgical calendar). After reading from the scheduled Tanakh readings, the Christians would add brief portions from one of the gospels or epistles that “interpreted” the Old Testament reading. Marcion got his “rule” of books from these additional Christian readings.
The leaders of the Church began to investigate as to whether any of their books were inspired. To do so they set their own “rule,” that it basically had to be apostolic in nature or written under the auspices of the same. The books had to be those commonly read in liturgical services across the world. It would take 200 years after Marcion before the Church had the New Testament “rule” or “canon” settled and the issue closed.
The books were not chosen by popularity or merely because they were quoted by other authors. For instance, Paul quotes from Greek poetry in Acts but this doesn’t mean that the writings of the Greeks is inspired. (Acts 17:28) Therefore just because there is a quote from a book like Enoch does not mean that it is inspired. Also, it appears Luke was added to prove Marcion wrong. Luke’s gospel is not apostolic, but it is very thorough, and it follows the same sources of Matthew and Mark (Mark wrote under the auspice of Peter, or so the understanding has been). There are also more letters from Paul than any other apostle, though he was not one of the original twelve and even history testifies that Petrine authority was recognized. This choices may have been to counter Marcionism.