In the West, the pope came to be a political power in his own right. He had his own base of power separate from the political rulers of the age. In the East, the Patriarch of the Orthodox Church tended to be a creature of the Emperor, lacked any independent power base, and supported imperial policy. It was also unique, for Arabia was a far away world to the west, and Mohammed's interpretation of God while it rested on a Judeo-Christian, Hellenistic base, was quite remarkable. The cities were in decline, especially in the West.
Cities depend on trade, on transportation and economic complexity to assert their importance.
But in the 400s, they simply became magnets for invading armies to attack and sack. With the city populations declining, there was much less profitability in the large estates of the late Roman world. The free farmers who had once made up the Roman Legions were much less important in their military roles. The success of mounted troops against foot soldiers meant that armies of the middle ages would be dominated by horse soldiers who could afford the expense of going to war. The small, independent farmers, able to exploit technological innovations in agriculture were no longer expected to take months off to fight.