Why were most ancient cities constructed like Burgess concentric model? CBD?

In 1923, E.W. Burgess, along with his two colleagues Robert Park and Roderick Mckenzie, developed the concentric zone model of city development. Their premise was that the internal structure of cities was based on the arrangement of social groups in a series of rings. The CBD, central business district, in Burgess model was at the center of the radiant circles. The lower classes lived nearer the center of the city, i.e., the CBD, than the upper classes because, presumably, the upper classes could afford the higher rents in the “suburbs,” and the expense of travel to do business in the CBD.  As the social class gained wealth, they moved into the next of the 5 transition zones.

On the one hand, ancient cities of the Roman Empire were built in a similar circular pattern but in relatively large open spaces without walls. The city design was that of two roads crossing in the city’s center and dividing the city into 4 sections. These two major thoroughfares were joined by many smaller roads that allowed for the flow of people throughout the city. The "tributary" roads formed the pattern of concentric circles. Efficiency was a key issue in developing cities that would be crowded with people, merchants, horses, carriages, wagons and military personnel. On the other hand, after the fall of Rome, medieval cities were in constant danger from invaders and war so they built walls around their cities for protection. There were only a few gates for entry to and exit from the city. With people confined to the area inside the city walls, and the market at the center, city developers used special flow structures, i.e., circular/radial flow, for optimal movement of goods and people. The concentric circular pattern would also lead to more efficient evacuation when necessary. 

The circular pattern of ancient and medieval cities had little to do with social hierarchy, though it is historically appropriate to presume the division between social classes was distinct and obvious. It had more to do with the convenient flow of traffic in the congested confines of the city, whether they were the more open Roman cities or the medieval walled cities. In addition, with the CBD at the center of town, no one had to traverse the whole city to reach the markets and other important business places.