Why was Jesús laid in a manger?

Jesus did not "be born inside" the manger; He was laid there after being born (Luke 2:7). That communicates a powerful message:

Humility: the King came without luxury, without privilege.

Closeness: His arrival was accessible to common people (shepherds) (Luke 2:10–12).

Clear sign: the manger was the "indicator" to recognize Him (Luke 2:12).

And it also connects with the prophetic plan: the Messiah would be born in Bethlehem (Micah 5:2; Luke 2:4–7).