Because the capital of the Roman Empire was transferred from Rome to Constantinople in 330 AD.
The Ecumenical Patriarch (in Constantinople).
Jesus started the First Church which was the Catholic Church. Martin Luther started the Lutheran Church.
Constantinople was the capital of the Eastern Orthodox Church.
This is the name given to the Eastern Orthodox Church where the Ecumenical Patriarch resides in Constantinople (present-day Istanbul, Turkey). It is the church founded by St Andrew the Apostle, and was the Cathedral Church during the Eastern Roman or Byzantine Empire. It is similar to saying the papal church of Rome where the Pope resides. The Ecumenical Patriarch in Constantinople is known as the 'first among equals' in the Orthodox Church.
Back in the days of Constantinople, the Orthodox Church was highly involved in government and politics. Even though there was separation of Church and State, the Church exercised considerable influence in the government of Constantinople.
The catholic church was never moved to constantinople, emperor constatine kept the church in Rome but moved the capitol to Constantinople. The SPRQ's belief of the church for salvation and emotional comfort kept the eastern empire alive for as long as it did, though it ultimately fell.
It was and is in Constantinople (Istanbul).
The Emperor
The Patriarch is the authoritative figure for The Greek Orthodox Church.
Princess Olga of Kiev was the first Russian leader to convert to Christianity. She was baptized in Constantinople into the Orthodox Church and is considered an equal to the Apostles.
The Eastern Orthodox Church in Constantinople is called the Ecumenical Patriarchate, or is sometimes called the See of St Andrew (because it was founded by St Andrew).
It was the Orthodox Church, or The Eastern Orthodox Church.