Why did Mary lay the newborn Jesus in a manger?

Answer: There was no room in the inn

Mary wrapped her firstborn in swaddling cloths and laid him in a manger, because there was no room for them in the inn.

“And she gave birth to her firstborn, a Son. She wrapped Him in swaddling cloths and laid Him in a manger, because there was no room for them in the inn.”— Luke 2:7 (BSB)