Because he wouldn't recognize his authority as a pope.
If the place is Constantinople then the answer is: because he wouldn't recognize his authority as a pope.
The Eastern Orthodox Church.
Pope Benedict IX
The last to be elected when not yet a bishop was Pope Gregory XVI in 1831, and the last to be elected when not even a priest was Pope Leo X in 1513.
The debate that prompted Pope Gregory II to excommunicate Byzantine Emperor Leo III revolved around the issue of iconoclasm, specifically Leo's edict against the veneration of icons and his efforts to remove religious images from churches. The pope viewed this as a direct challenge to the authority of the Church and a threat to traditional Christian practices. Gregory II's excommunication of Leo was a significant moment in the growing divide between the Eastern Orthodox Church and the Roman Catholic Church, highlighting the tensions over religious authority and the role of images in worship.
Patriarch Leo of Constantinople died in 1143.
I think pope Leo I
Two Popes that declared the other to be a false pope, excommunicating each other.French pope lived in Avignon and the other Italian pope lived in Rome, this began the division between the ChurchesA:The Council of Nicaea, called by the Roman Emperor Constantine in 325, decided that Christianity in the Roman Empire would be led by four senior bishops or Metropolitans representing Rome, Alexandria, Antioch and Jerusalem. The Council also made provision for Constantinople and Carthage, where Metropolitans were subsequently appointed, although the role was short-lived in Carthage. Thus, religious authorities in the mainly Greek-speaking eastern regions held that the bishop of Rome was one among equals. However, the bishop of Rome, designated in the west as the Pope, insisted that he had greater authority than the other Metropolitans. Gradually, differences built up until, in 1054, Pope Leo IX insisted on the right to make a change to the Nicene Creed, on his own and apart from an Ecumenical Council. The Great Schism resulted when Pope Leo IX sent envoys who purported to excommunicate Patriarch Michael I, who in turn excommunicated the envoys, Leo having died just prior to this meeting.
Two Popes that declared the other to be a false pope, excommunicating each other.French pope lived in Avignon and the other Italian pope lived in Rome, this began the division between the ChurchesA:The Council of Nicaea, called by the Roman Emperor Constantine in 325, decided that Christianity in the Roman Empire would be led by four senior bishops or Metropolitans representing Rome, Alexandria, Antioch and Jerusalem. The Council also made provision for Constantinople and Carthage, where Metropolitans were subsequently appointed, although the role was short-lived in Carthage. Thus, religious authorities in the mainly Greek-speaking eastern regions held that the bishop of Rome was one among equals. However, the bishop of Rome, designated in the west as the Pope, insisted that he had greater authority than the other Metropolitans. Gradually, differences built up until, in 1054, Pope Leo IX insisted on the right to make a change to the Nicene Creed, on his own and apart from an Ecumenical Council. The Great Schism resulted when Pope Leo IX sent envoys who purported to excommunicate Patriarch Michael I, who in turn excommunicated the envoys, Leo having died just prior to this meeting.
because he decided to start a fake religion
Pope Leo XIII was the pope in 1896. He was pope from 1878 to 1903.