Nicaean Canon

Lately I have been reading through the seven Ecumenical Councils. These are the councils in which all of orthodoxy agree with. All except most Protestants, I think some of us would deny the council just for the sake of denial. Anyway, when I started reading, I thought the Council of Nicaea (325 AD) created the Nicaean Creed, excommunicated Arius, and that was it. But lo and behold, there are about twenty canons that was also given with lot of incite on the history of the church. The one I have been looking at lately is canon six.

Let the ancient customs in Egypt, Libya and Pentapolis prevail, that the Bishop of Alexandria have jurisdiction in all these, since the like is customary for the Bishop of Rome also. Likewise in Antioch and the other provinces, let the Churches retain their privileges. And this is to be universally understood, that if any one be made bishop without the consent of the Metropolitan, the great Synod has declared that such a man ought not to be a bishop. If, however, two or three bishops shall from natural love of contradiction, oppose the common suffrage of the rest, it being reasonable and in accordance with the ecclesiastical law, then let the choice of the majority prevail.

Along with the Ancient Epitome

The Bishop of Alexandria shall have jurisdiction over Egypt, Libya, and Pentapolis. As also the Roman bishop over those subject to Rome. So, too, the Bishop of Antioch and the rest over those who are under them. If any be a bishop contrary to the judgment of the Metropolitan, let him be no bishop. Provided it be in accordance with the canons by the suffrage of the majority, if three object, their objection shall be of no force.

It seems, and I am no authority, that the church had an idea. That is, let's separate the diocese according the government in charge! But when Rome conquered all, the Roman Bishop became head of all. So, why did he stay in charge when Rome fell?