In the Gospel of Mark we do not find an Infancy Narrative like we do in the Gospels of Matthew and Luke. Why? I don’t know that we can find a definite answer, but this is a collection of possibilities. Some are not exclusive of the others.
(1) Mark didn’t know about the virgin conception. It may be that for one reason or another he was unaware of this story. It is hard to include something you don’t know!
(2) The virgin conception had not been ‘created’ before the composition of Mark’s Gospel. If Matthew was the originator of the story this would make sense. Mark cannot know what Matthew created. If Matthew did create it we must explain Luke’s knowledge of it. This would demand Luke’s knowledge of Matthew or a tradition derived from Matthew.
(3) Mark rejected the virgin conception. If he was aware of it he may have not affirmed it. This would put him at odds with Matthew and Luke, though maybe not John.
(4) Mark may have sought to avoid controversy regarding the birth of Jesus. It is apparent that some felt that Jesus was the result of Mary’s sexual immorality. This seemed hinted at by the Gospel of John 8.41 where Jesus’ opponents state, “We were not born of fornication.” Did they mean to insinuate that Jesus was? Whether this was true or not it could have been sufficient reason for Mark to avoid any discussion of Jesus’ birth or childhood lest it distract his audience, especially if he knew his audience was aware of these accusations.
(5) The virgin conception was irrelevant to the theme(s) of Mark’s Gospel. In Mark 1.1 we read “The beginning of the gospel of Jesus Christ, the Son of God.” In 1.2 he moves straight into the proclamations of John the Baptist. Mark ends abruptly with a resurrection narrative. It could be that Mark’s main concern was the “ministry” of Jesus from the preaching of the Baptist to the resurrection. Somehow, for Mark, this is “the gospel” and Jesus’ birth is foreign to “the gospel”.
(6) The virgin conception is assumed. If the title ‘Son of God’ had obtained a high Christology by the time Mark wrote his Gospel then maybe calling Jesus Christ the ‘Son of God’ rather than the ‘son of Joseph’ was enough of an introduction. Of course, this depends on whether one thinks ‘Son of God’ was original with Mark’s Gospel in 1.1. The same could be said of John’s Gospel with its logos Christology. There may not have been any reason to discuss a virgin conception when you are claiming that God’s Word became flesh and tabernacled among humans (1.14).
(7) Matthean Priority is accurate. If Mark knew Matthew’s Gospel was in circulation and he was seeking to create a shorter, catechism of a Gospel then he may not have felt like rehearsing what was already declared in a longer, earlier Gospel. This could pair with possibility #3.
Did I miss any of the possible explanations?