According to Dave Armstrong:
The Orthodox decided to split off. That was simply yet another instance of the constant schismatic (as well as caesaro-papist) tendency of the East. After all, they had done so at least five times before in the previous 700 years, and were on the wrong side of the debate in every case (231 out of 500 years, or 46% of the time!), according to their own judgment now (and our Catholic standard):
The Arian schisms (343-398)
The controversy over St. John Chrysostom (404-415)
The Acacian schism (484-519)
Concerning Monothelitism (640-681)
Concerning Iconoclasm (726-787 and 815-843)
These are historical facts, that can be easily verified. Anyone can go look it up if my report isn't trusted. For much more along these lines, see my paper, A Response to Orthodox Critiques of Catholic Apostolicity .
Secondly, if an Orthodox wishes to claim primacy, then he has to show that his doctrines are that of the early Church, over against Catholic doctrines. But they clearly are not, in several clear instances. The most clear ones are in the case of the papacy (we continue to have it like the early Church; they do not), ecumenical councils (we continue to have them like the early Church; they do not), divorce (we continue the overwhelming patristic consensus on no divorce and no remarriage; they do not, and this first changed in the sixth century in the East), and contraception (they now widely sanction it; we continue to regard it as grave sin, as all Christians did until 1930, including the fathers, as contraception was not unknown at all in ancient times).
Take your pick. If one desires apostolic Christianity: the Christianity of the apostles and Church fathers, there is no contest: Catholicism is for you. Orthodoxy caved to Byzantine cultural pressure in the sixth century, to change the apostolic and patristic teaching on divorce and indissoluble marriage, and it caved into the sexual revolution and modernity in the last fifty years, to change its views on contraception and allow what it once regarded as a grave sin, while Catholic teaching in both regards remains as it always has from the beginning.
I look forward to the deluded souls over at Energetic Procession, Well of Questions, Ad Orientem, &c., swimming the Tiber now that dear old Dave has definitively shown them the error of their ways.