Alexandria was a major city during the Roman period, the leading city in Egypt and North Africa, a site of considerable learned communities, and had a very early Christian presence that self-identified its founding with Mark the Apostle.

Antioch likewise was one of the major population centers and leading cities of the east, it had a very early Christian presence, as recorded in the Acts of the Apostles, and played an early and significant role in Christian growth in the area.

Carthage did not share these features to the same extent. That is, while Roman Carthage did grow to become a major population centre, its Christian presence didn't have the antiquity that the penarchate sees did, nor a direct link to an apostolic or New Testament figure. That, and the fact that, like all Latin speaking areas, it functionally (and then formally) ceded authority to Rome, meant that it never had the ecclesio-political clout to be recognised as part of the Pentarchy.

Early Christian expansion was predominantly in the Greek speaking Eastern side of the Empire/Mediterranean. The largest cities in the Roman Empire were (1st century CE) Rome, Ephesus, Alexandria and Antioch.

Therefore, these cities were both more important and had larger christian communities since the early days of christianity (since they were early target of apostolic labor, St Mark going to Alexandria and Peter to Antioch).

By the time the idea of the Pentarchy came to be, under Justinian, Ephesus was now in the sphere of Constantinople and not the other way around. Antioch, as the provincial capital of Syria and Alexandria, same role for Egypt, continued being highly influential and with large and deep rooted christian communities.