Why did Joseph and Mary have to go to Bethlehem to register for a census?
The Roman custom was to register in one’s own domicile because the pagans did not care about their ancestry whereas the Israelites kept their genealogy tracing back to the 12 sons of Jacob. Since they had to register the census according to their tribes and clans, they had to go to their own native land.
Joseph, the foster father of Jesus, was a descendant of David’s son Solomon. Mary was a descendant of King David’s son Nathan. So, we have two genealogical accounts of Jesus through the ancestral lines of Joseph by Matthew and of Mary by Luke. These show that Jesus is the “Son of David” from both by blood (Mary) and by right (Joseph).
Since the ancestors of Joseph and Mary were from Bethlehem, the birthplace of David, they had to go there for the census. They travelled over 150 km from Nazareth to Bethlehem, a journey that would have definitely taken many days.