Why do the NT books appear where they do in the Bible?
Of course it made sense for the four gospels to be placed first, because Matthew, Mark, Luke, and John each in his own way presented the life of Christ who was, Christians believe, the God-man who from earthly perspective founded the religion now known as Christianity. It made equally good sense to place the book of Acts next, because it was the only history book of the first generation of Christianity after the death and resurrection of Jesus. Except for the book of Revelation, the remaining NT documents are all letters or epistles. It makes sense to group them together according to their authors and because Paul was the most prolific epistle writer and arguably the most important in the life of the early church and the influence of the apostolic documents, Paul's letters were placed first.
The second half of the book of Acts primarily focuses on the missionary journeys of the apostle Paul and so here we find another reason for placing Paul's letters immediately following the book of Acts. Then comes the epistle to the Hebrews which in the earliest manuscripts for many centuries had no ascription of authorship attached to it at all. From the earliest years of church history Christians debated who the author of Hebrews was. Some argued for Paul, but as we will see when we come to our introduction to Hebrews several other candidates were put forward as well and early church fathers simply admitted ignorance of this letter's author. Because of the feeling that in some circles that there was a connection with Paul, Hebrews was placed after the letters attributed to Paul, but it was not initially identified as coming from him.
Then come letters from four early Christian leaders, James, Peter, John, and Jude and as best as we can tell they were put in that order because of the prominence of those four men in the earliest decades of the Christian movement. Of course, it will come as a surprise to some people that James was initially a more central and foundational Christian leader then Peter, but this appears to be what Acts in it's first fifteen chapters discloses as James is described as the chief elder of the church in Jerusalem, even as Peter quickly becomes a more itinerate minister due to the persecution of Christians in the most heavily Jewish portions of the Roman Empire.
Peter, however, remains a very significant early Christian leader. His right hand man or side-kick, if we like, is regularly described to be the apostle John and Jude is clearly the least significant of the four. More will be discussed about these individual apostles when we introduce the letters they wrote, one at a time. Revelation, being of an entirely different genre, a work of apostolic but also about the end of history as we know it and probably written last of all the canonical Christian documents, naturally fell at the end of the NT canon.
It is important to stress that this organization did not occur immediately and without various other orders proposed and used in the earliest documents of the Christian era, but when the canon was standardized, largely by the fourth century, more and more this sequence came to prevail.