Joseph & Mary went to Bethlehem because it was the "City of" this king & Joseph came from his lineage
... And 
Joseph also went up from Galilee, out of the city of Nazareth, into Judaea, ... was 
of the house and lineage of David:) To be taxed with Mary his espoused wife, ... 
Bethlehem of Judaea in the days of Herod the king, behold, there came wise men
... [DOC] [TLE] "Where did Joseph and Mary live before the birth of Jesus?"