Why was Martin Luther attempting to change the canon?

It might be much better if you re-worded your question. Luther was never seeking to change the canon. Instead, it might be better to simply say that Luther was seeking to find the canon. Very early in the Christian church there were false-teaching men who pretended to be Christians, but clearly did not teaching and preach what was handed down to the apostles by Jesus (Valendinius, Cerinthus, Arius, et multa.) As a result of these false teachers there were different categories of books floating around the Mediterranean world. I will explain them as follows:

• Homologumena. These are the books that are clearly written by the apostles and belong in the bible. Schaff has this able description of them:

The principal books of the New Testament, the four Gospels, the Acts, the thirteen Epistles of Paul, the first Epistle of Peter, and the first of John, which are designated by Eusebius as “Homologumena,” were in general use in the church after the middle of the second century, and acknowledged to be apostolic, inspired by the Spirit of Christ, and therefore authoritative and canonical. This is established by the testimonies of Justin Martyr, Tatian, Theophilus of Antioch, Irenaeus, Tertullian, Clement of Alexandria, and Origen, of the Syriac Peshito (which omits only Jude, 2 Peter, 2 and 3 John, and the Revelation), the old Latin Versions (which include all books but 2 Peter, Hebrews, and perhaps James and the Fragment of Muratori

Philip Schaff, Ante-Nicene Christianity (History Of The Christian Church 2; Accordance electronic ed. 8 vols.; New York: Charles Scribner’s Sons, 1910), n.p.

• Antilegomena. These are the books that are spoken against. Now, here the important detail to understand is that they were not spoken against because they didn't belong in the bible. They did belong in the bible. But there was a question as to why they belonged in the bible. Revelation, with its vision-filled content, Hebrews, with its unknown author—these were spoken against because there were details they wanted to know about them. But it was clear that they were part of the canon already. Again, Schaff:

Concerning the other seven books, the “Antilegomena” of Eusebius, viz. the Epistle to the Hebrews,943 the Apocalypse,944 the second Epistle of Peter, the second and third Epistles of John, the Epistle of James, and the Epistle of Jude, — the tradition of the church in the time of Eusebius, the beginning of the fourth century, still wavered between acceptance and rejection. But of the two oldest manuscripts of the Greek Testament which date from the age of Eusebius and Constantine, one — the Sinaitic — contains all the twenty-seven books, and the other — the Vatican — was probably likewise complete, although the last chapters of Hebrews (from Heb. 11:14), the Pastoral Epistles, Philemon, and Revelation are lost.

Philip Schaff, Ante-Nicene Christianity (History Of The Christian Church 2; Accordance electronic ed. 8 vols.; New York: Charles Scribner’s Sons, 1910), n.p.

• Pseudepigrapha: These are the falsely-attested writings. In other words, many non-Christian writers wanted to spread their beliefs so they attached the name of an apostle to their writings to give them credence. The most notable in this category today are the Gnostic gospels like the Gospel of Thomas and the Gospel of Mary.

All of this is vitally important to know in answering your question, Richard, because, by the time Luther came along, the western church had added many books to the bible that the church had never considered part of the bible. And Luther faced the difficult task of testing them to see if they belonged in the bible or not. Some of the apocryphal books (e.g. Maccabees), he considered interesting and useful in some contexts, but not part of the bible. The last group he wrestled with was the antilegomena. His struggle with James is well-known. But where he ended up is clear. In his prefaces to the books of the bible, in the beginning, he had no use for James. But in prefaces to his final editions, before his death, he admitted that it belonged in the bible.

Conclusion

Luther never changed the canon. Instead, it would be better to say that he rediscovered it. And he rediscovered it not by taking out a blow-torch and a machete and cutting out parts the bible, but instead studying the bible and comparing the books of the bible with themselves first of all. Then he looked at what the early Christian church used and how they evaluated the books that were in the bible and the books that were outside of the bible.

Pastor Steve Bauer (http://stevebauer.us)