Why did Mary and Joseph travel to Bethlehem?

Answer: For a census

Caesar Augustus issued a decree for a census, so everyone went to their own town to register.

“And everyone went to his own town to register. So Joseph also went up from Nazareth in Galilee to Judea, to the city of David called Bethlehem, since he was from the house and line of David.”— Luke 2:3-4 (BSB)