I have always wondered why the canonical Gospels were organized in the traditional Matthew-Mark-Luke-John order in the New Testament. It seems logical to me that John should come last since it is so different from the synoptics (which should be "seen together") and probably was the last of the Gospels written. I have read that Matthew received the "pride of place" due to its wide use in church government and liturgy. But these two insights do not explain the overall order of the Gospels.
The order is not alphabetical by author's name. Neither does the order appear to be chronological. Most contemporary scholars see Mark as the first written Gospel with Matthew and Luke using Mark when they constructed their later Gospels. There are alternative theories - none of which I find convincing - that put Matthew first. But none - to my knowledge - argue for a Matthew-Mark-Luke-John chronological order.
So to this day, I am not sure why the Gospels appear in the Matthew-Mark-Luke-John order.
Now to make matters worse, I have realized that I have exactly the same lack of understanding about the order of Paul's letters in the New Testament. II Peter implies that Paul's letters may have been collected long before the closing of the NT canon. So it is altogether possible that the order of the letters were determined prior to their inclusion in any of the early "canon" lists.
Paul's letters were not ordered chronologically. Romans - which appears first in the NT - was certainly not the first Pauline letter written. In fact, Romans appears to come late in Paul's story as he plans a missionary trek to Spain that will necessarily lead him first to Rome.
There does appear to be some topical groupings in Paul's letters. Certainly, the pastoral letters - I and II Timothy and Titus - fit together nicely in tone and content. The "prison" letters to the Ephesians, Philippians, and Colossians seem to be written later in Paul's life, each reflecting a sense of imprisonment and a maturity in Paul's theological thinking. But topical grouping breaks down with the other letters.
Why is Romans first on the list of Paul's letters? Many would argue that Romans is the most important of Paul's letters, his most comprehensive presentation of the gospel, almost a "systematic theology" of Paul's thinking. But if the doctrinal preeminence of Romans pushed it to the top of the list, certainly the other doctrinal letters - Galatians and Philippians - should appear early on the list as well, but they do not.
This week, I accidentally bumped into a convincing answer to my questions while reading a commentary on the Thessalonian letters. In a passing comment - a little throwaway sentence - that makes everything clear about the ordering of Paul's letters, I found the answer. This insight was so obvious, so apparent, that I could not believe I had not seen it before.
Paul's letters are organized by their length - their physical size. Beginning with Romans - Paul's longest letter - and ending with Philemon - Paul's shortest (one chapter) letter - Paul's letters decrease in size as they are ordered in the NT.
Sometimes you can't find a deep theological explanation of things because none exists. Sometimes a simple, obvious explanation is best.