I understand that the original canonical gospels (Matthew, Mark, Luke, and John) were written for different audiences, which makes sense. However, why hasn't the Church since combined the four into one document?
They were written to different audiences and written at different times for different purposes.
The manuscript under the name of Mark, written most likely within 15-20 years of the crucifixion, was succinct and unembellished to mark the life and death of Jesus. There is no infancy narrative and no material following the crucifixion. The original narrative began with the prophecy of John the Baptist and ended at the empty tomb. Most historians agree that Mark 16:9-19 was tacked on later to be a reassurance of the resurrection. This addition could have been by the same writer or somebody else.
Virtually all of the content of Mark is found in Matthew so it was almost certainly written after Mark but before the destruction of the Temple in 70 a.d. Matthew added material that Mark left out with a primary purpose to emphasize that Jesus is the Messiah, the Son of God, and fulfillment of prophecy. It begins with Jesus's genealogy from Abraham to Joseph, the husband of Mary, of whom was born Jesus, who is called Christ. It ends after the resurrection with Jesus giving the Great Commission to the eleven remaining Disciples.
Luke also includes virtually all the content of Mark so it was also almost certain written after Mark but again, based on historical references in the content, before the destruction of the Temple in 70 a.d. Luke was fascinated by the miracles and miraculous concepts and the stories that demonstrated them. He too begins with the prophecy and miraculous circumstances of John the Baptist's birth and how it was inextricably intertwined and predestined with Jesus's own. He also included a genealogy that closely paralleled Matthew's up to King David and then was completely different suggesting he was giving Mary's lineage as Jesus's 'biological' Father was God and not Joseph--more emphasis on the miraculous circumstances of the birth, life, death, resurrection and continuing into the Book of Acts, the establishment of Jesus' Church.
Both Matthew and Luke vary a bit from the content of Mark, no doubt because all three were drawing from at least some different witnesses. Matthew contains material found nowhere else in the Bible. Luke contains materials found nowhere else in the bible. And while there are also many areas of agreement between Matthew and Luke and/or among all three, they do not parallel each other. In no place do Matthew and Luke agree with each other against Mark.
And John was almost certainly written after the destruction of the Temple in 70 A.D., possibly close to the end of the century, and is almost certainly written as inspiration of hope for a Church under siege and under threat. It focuses on Jesus as the Son of God on Earth given for our sins and also Jesus the Christ who is God who loves and cares for us and offers hope when things are at their worst. It is so different that it cannot be even studied synoptically let alone incorporated with the other three Gospels.
Having Four Gospels is much easier to study, digest, understand, and be blessed by than one huge enormous manuscript would have been.
