Mark wrote a relatively short gospel. He started with Christ's earthly ministry. It seems that Christ's nativity was not that important to his point of view. Or, perhaps, he knew that Matthew planned to expand on his writing. That Mark choose not to write on Christ's nativity ... doesn't mean that there wasn't a nativity ... as we all have one.
There were (3) ways that the gospel writers introduced their narratives on Christ's life/ministry. Matthew and Luke included the earthly nativity, John started back at the beginning (of the creation), and Mark started with the beginning of Jesus' earthly ministry. Matthew began his gospel with an accounting of Jesus' genealogy, ... while Luke started his gospel with an explanation of why he wrote it, ... followed by an accounting of John the Baptist's nativity.
The gospel writers had different reasons (and audiences) for their writings. Luke, who wrote the longest gospel, ... emphasized in his introduction that he wanted to try and present every bit of vital information for an, at least, partially non-Jewish audience.
Luke 1
1 Many have undertaken to draw up an account of the things that have been fulfilled among us, 2 just as they were handed down to us by those who from the first were eyewitnesses and servants of the word. 3 With this in mind, since I myself have carefully investigated everything from the beginning, I too decided to write an orderly account for you, most excellent Theophilus, 4 so that you may know the certainty of the things you have been taught.
(Note that Luke also included the nativity of John the Baptist, ... when none of the other writers do.)
Also, recall that the gospel writers had, more or less, differing connections to the ministry of Jesus. Matthew and John were actual Apostles, while Mark was a disciple of Peter, ... and Luke was the Greek companion/physician of Paul. So they all come to the story in different ways ...