There are several reasons. After the eastern and western Roman empires split, they each formed their own popes. West Rome is a Roman Catholic church, which we call Catholicism, and East Rome has formed the Orthodox Church, which is the Orthodox Church. Under the management of the Roman emperor, the Pope was only a subordinate official of the emperor. However, with the collapse of the Western Roman Empire, the situation changed, the Roman state was destroyed, the so-called barbarians entered the historical stage, and a large number of Germanic tribes entered the hinterland of Europe. Then the Frankish kingdom conquered most areas, and its barbarian emperor accepted Christianity. In order to legally accept the rights of the Roman Empire, Emperor Frank adopted the coronation ceremony of the Pope and inherited the legal rights of the Roman Emperor. Later, with the division of the Frankish Empire, Europe entered a period of competition among divided vassal States, and the church experienced a qualitative change:

First of all, in order to get the legitimate throne, the vassal state naturally formed the necessity of the pope's coronation, and the vassal state no longer had the strength of the Frank emperor, so the initiative was transferred to the church;

Secondly, in the process of the competition for imperial power in the Middle Ages, the princes fought constantly, and the ordinary people lost hope, so they gave all their hopes to God and worshipped the church more closely, hoping that Christ would come again and save the people in from the mire.

Third, the feudal system of European nobles divided the land smaller and smaller, and religion had no boundaries. As the area ruled by the Lord became smaller and smaller, the church became bigger and bigger, gradually making the Pope above the royal power, and the whole of Europe was finally shrouded in theocracy. The corresponding Eastern Roman Empire was not destroyed by the Turkish Empire until 1453, so the churches of the Eastern Roman Empire have always been in the same position.