Why do the Protestants believe the Bible alone to the the sufficient source of Divine Revelation?

Why do the Protestants believe the Bible alone to be the sufficient source of Divine Revelation? When the books composing the New Testament were written, the various writers had some definite and immediate purpose in mind. St. Matthew wrote a Gospel to show that Jesus is the promised Messiah. St. Mark, a disciple of St. Peter, wrote a Gospel to show that Christ is the Son of God. St. Luke, converted by St. Paul, wrote a Gospel for a distinguished citizen of Rome, Theophilus, to instruct him in the faith of Christ. St. John wrote a Gospel to prove against the heretics that Jesus is true God. St. Paul wrote his Epistles to the various churches of Gentiles that he had converted, to remind them of his teachings in his absence. Not everything was written down about Jesus. St. John says, "There are, however, many other things that Jesus did; but if every one of these should be written, not even the world itself, I think, could hold the books that would have to be written." (John 21:25)