On 22 October 2011, the great archimandrite of the Ecumenical Throne, Athenagoros celebrated the Divine Liturgy at the Patriarchal Church of St. George the Victorious in the Phanar to mark the 20th anniversary of the election and enthronement of Patriarch Bartholomew of Constantinople...
Any idea why they did not serve?
who in particular are you asking about?
also, at the patriarchate it is routine that only 1 priest will serve, with at least 1 deacon (usually 2, and up to 4). They do this out of a sense of simplicity, they like it when it's simple. When I was there, there was not 1 co-celebration & there were like 3 feast days, including the Patriarch's name day. Each time, only 1 priest served, the patriarch was at the throne, and at least 2 deacons served each time. That's just the way they do it.