This question is hard to answer directly. Simply put, St. Mark’s Gospel doesn’t contain as much material as those Gospels attributed to St. Matthew, St. Luke or St. John.
When comparing Mark’s with the other canonical Gospels, particularly the other synoptics, it is obvious that that there are various stories or episodes from the life of Christ that are left out of his. Also, while Matthew and Luke record details of Jesus’ birth and infancy and his genealogy, Mark’s begins with Christ’s baptism — at the inauguration of his public ministry.
Jesus’ own words are more limited in Mark’s Gospel as well. What was recorded is not as lengthy or verbose, as would be in the case of a Gospel like St. John’s. There are not as many parables or teachings of Jesus appearing in St. Mark’s Gospel, but there is an emphasis on his role as wonderworker and healer.
St. Mark’s Gospel was once thought to be a summary of St. Matthew’s Gospel, although that it is no longer the opinion held by the majority of scholars, who are mostly in agreement today that St. Mark’s Gospel is the oldest — derived from the teaching of St. Peter, of whom St. Mark was a disciple.
The Gospels were written primarily for an already Christian audience and secondarily as a tool for evangelization. The Gospels would therefore have had a limited audience at first, given Christianity’s status within the Roman Empire. Since St. Mark’s is considered the oldest Gospel, it makes sense that he would not have necessarily included details that would have been more important to those needing convinced that Jesus was Lord.
And when speaking of the brevity of St. Mark’s Gospel, it is significant to indicate that its original form is believed to have been even shorter. First ending with the discovery of the empty tomb alone, there is an addition to the end that included a post-narrative of the resurrection.
Michael R. Heinlein is editor of Simply Catholic.