The answer
Because these three Councils (Nicea, Constantinople, and Ephesus), took place before the division of the church.
H.G. Bishop Moussa
H.G. Bishop Moussa · Youth Concerns · 22
Faith Q&A › Youth Concerns › Theology, Doctrine, and Dogma
Why does the Coptic Church only recognise three Ecumenical Councils?
H.G. Bishop Moussa, Bishop of Youth, Youth Concerns (Questions & Answers). Compiled by Lisa Agaybi. Coptic Orthodox Patriarchate, Bishopric of Youth Bookstore.