Why do so many think the Bible Canon was decided at the Council of Nicea?

It was decided a the Council of Rome 382 A.D.

ST. DAMASUS 1, POPE, THE DECREE OF DAMASUS:

It is likewise decreed: Now, indeed, we must treat of the divine Scriptures: what the universal Catholic Church accepts and what she must shun. The list of the Old Testament begins: Genesis, one book; Exodus, one book: Leviticus, one book; Numbers, one book; Deuteronomy, one book; Jesus Nave, one book; of Judges, one book; Ruth, one book; of Kings, four books; Paralipomenon, two books; One Hundred and Fifty Psalms, one book; of Solomon, three books: Proverbs, one book; Ecclesiastes, one book; Canticle of Canticles, one book; likewise, Wisdom, one book; Ecclesiasticus (Sirach), one book; Likewise, the list of the Prophets: Isaiah, one book; Jeremias, one book; along with Cinoth, that is, his Lamentations; Ezechiel, one book; Daniel, one book; Osee, one book; Amos, one book; Micheas, one book; Joel, one book; Abdias, one book; Jonas, one book; Nahum, one book; Habacuc, one book; Sophonias, one book; Aggeus, one book; Zacharias, one book; Malachias, one book.

Likewise, the list of histories: Job, one book; Tobias, one book; Esdras, two books; Esther, one book; Judith, one book; of Maccabees, two books. Likewise, the list of the Scriptures of the New and Eternal Testament, which the holy and Catholic Church receives: of the Gospels, one book according to Matthew, one book according to Mark, one book according to Luke, one book according to John. The Epistles of the Apostle Paul, fourteen in number: one to the Romans, two to the Corinthians, one to the Ephesians, two to the Thessalonians, one to the Galatians, one to the Philippians, one to the Colossians, two to Timothy, one to Titus one to Philemon, one to the Hebrews. Likewise, one book of the Apocalypse of John. And the Acts of the Apostles, one book.

Likewise, the canonical Epistles, seven in number: of the Apostle Peter, two Epistles; of the Apostle James, one Epistle; of the Apostle John, one Epistle; of the other John, a Presbyter, two Epistles; of the Apostle Jude the Zealot, one Epistle. Thus concludes the canon of the New Testament. Likewise it is decreed: After the announcement of all of these prophetic and evangelic or as well as apostolic writings which we have listed above as Scriptures, on which, by the grace of God, the Catholic Church is founded, we have considered that it ought to be announced that although all the Catholic Churches spread abroad through the world comprise but one bridal chamber of Christ, nevertheless, the holy Roman Church has been placed at the forefront not by the conciliar decisions of other Churches, but has received the primacy by the evangelic voice of our Lord and Savior, who says: "You are Peter, and upon this rock I will build My Church, and the gates of hell will not prevail against it; and I will give to you the keys of the kingdom of heaven, and whatever you shall have bound on earth will be bound in heaven, and whatever you shall have loosed on earth shall be loosed in heaven."

This false idea about Christian history was popularized by Dan Brown's novel , The Da Vinci Code. It has been debunked by scholars. See for example Errors 1, 2 and 3 listed in the review of DVC in Biblical Archeology Review (link below). The Council of Nicea didn't even discuss the Canon (list of authoritative books) of the Bible. The list of issues decided at the First Council of Nicea (325 AD) can be found in the Catholic Encyclopedia on the New Advent site. The CE is considered a scholarly source.

The last words of the New Testament were written by St. John who died in 100 A.D. Its many letters were scattered & first needed to be found, then verified and put into sequence. The shards & pieces of parchment & skin were in poor condition, but the living Doctors of the Church, Ephraem, Hilary, Augustine, Ireneus, Athanasius, Basil, Ambrose, Jerome, Chrysostom, for 300 years deciphered what the Apostles had preached, translated their sermons, and in 382 A.D. as JMR correctly states, the completed Vulgate, translated by St. Jerome, who was fluent in seven languages (many extinct at that time) was promulgated in perpetuity as the Gospel of Christ's Church, never to ever be changed.under pain of excommunication.

God teaches; Man conforms.

Traditional Catholics of Trent at traditionalmass.org/

You are fundamentally correct but Nicea laid the foundation for further councils.

Why do so many Christians think it was canonized during the first century, that would have been a much better question?

Like how people think the Pali Cannon is older than the Mahayana Sutras...It does not matter, what religions do for people is the most important.

I don't know really but many christians reject the catholic church decided the canon of the New testament