They are considered canonical for the same reason that any other writing is in the canon – because the Church discerned that they were inspired and included them in the Scriptures.
There is the canon of the OT – those writings that gained univeral acceptance early – and the “deuterocanon” – those writings about which there was some controversy in early Christian history. The deuterocanon of the NT consists of Hebrews, James, 2 Peter, 2 and 3 John, Revelation, and Mark 16:9-20. The “deuterocanon” of the OT are Tobit, Judith, Wisdom, Ecclesiasticus (Sirach), Baruch, 1 and 2 Maccabees, parts of Esther (10:4-16, 14) and Daniel 3:24-90, 13, 14).
“Deuterocanon” is a word used since the 16th century to identify those writings that were not universally accepted early on in the history of the Church, but were accepted at some later time. All of these are recognized by the Catholic Church as part of the biblical canon. They were all named to the canon at the very same time, at the Councils of Rome (A.D. 382), Hippo (393), Carthage III (397) and IV (419), and again at II Nicaea (787), Florence (1431-45), Trent (1545-63). Maccabees is as canonical as Matthew; Hebrews is as canonical as Isaiah.
Jim Dandy