Compare Jerome's position to Augustine wih regard to the Apocrypha. Compare the Protestants' position to that of the Apostles' with regard to the Deuterocanonicals.
As far as Jerome, he was persuaded to include the deuterocanonicals in his Vulgate since the books were so widely accepted by the Church. Refer to Jerome's defense of his inclusion of the parts of Daniel rejected by the Jews:
"What sin have I committed if I followed the judgment of the churches? But he who brings charges against me for relating the objections that the Hebrews are wont to raise against the story of Susanna, the Son of the Three Children, and the story of Bel and the Dragon, which are not found in the Hebrew volume, proves that he is just a foolish sycophant. For I was not relating my own personal views, but rather the remarks that they [the Jews] are wont to make against us" (Against Rufinus 11:33 A.D. 402).
Jerome acknowledged how the canon was settled: by the judgment of the Christian Church, not of the Jews (and note that some Jews still include the Deuterocanonicals). The Protestants wanting to follow these Jews in deciding which books to include in their Bible should be consistent and also reject the New Testament books they rejected.