The scepter had departed from Judah; the Romans were in control of Palestine, and the time for the birth of the mighty One was due.

Exercising his perfect wisdom and power, Jehovah was overruling all things to the accomplishment of his purpose.

Augustus Caesar, then the emperor and ruler over all Palestine, issued a decree that all the people should be taxed.

Every one must go to the city of his nativity, there to be numbered and taxed.

Joseph the carpenter, although a resident of Nazareth, was of the house of David, and hence must go to the city of David to be numbered and taxed.
Naturally his espoused virgin would accompany him to that city.