Here’s a bit of an article explaining it:
VATICAN CITY, DEC. 19, 2007 (
Zenit.org).- The unique Nativity scene in St. Peter’s Square is a reminder from our teaching Pope of a father’s importance in the family, says Christian art expert Elizabeth Lev. …
"It is trying to represent Joseph’s experience, his dream of what will happen knowing that Jesus will be born and Mary is with child. He had no idea the way it would all work out as he prepared to take on this task given to him by God through a message of an angel. So it is what he imagines in his home amid his work before it comes to pass …
"Joseph is never the star of the Nativity, but his role is crucial. So this is not upsetting biblical tradition, but [giving] a different facet, looking at it in a different light …
“… this Nativity makes present the importance of the father figure and the fact that he is essential. It’s a reminder that he wasn’t born only to a mother, while providing a source of meditation during this time when we are faced with the battle against marriage and the family …”
You can read the rest of the article here:
zenit.org/article-21335?l=english