It is not really until the second century and the rise of Christian gnostics who asserted that Jesus had not been present in an actual physical form — that he had been spiritual only — that Christian thinkers realized that they’d have to start emphasizing Christ’s bodily origins. And so they have to talk about the registration at Bethlehem, they have to talk about the cradle, and even the swadling clothes become an article of faith. So it’s after about a hundred years that Christians decide to start thinking seriously about the nativity, and so the next question for them is when to mark it.
Bowler, Gerry. 2016. The Persistence of Christmas: A Conversation with Historian Gerry Bowler. Accessed December 25. http://www.albertmohler.com/2016/12/19/christmas-gerry-bowler/
If you are thinking, “But but … weren’t the nativity stories written in the first century?” then have a look at Marcion and Luke-Acts.