Since Polycarp asked in another thread, I’ll take a stab at the question. First, I am going to restrict my answer to Canonical Eastern Orthodoxy. The Oriental Orthodox (non-Chalcedonian) are better equipped to answer this question for themselves–I haven’t a clue given their organization over here.
So, the following list is a (probably incomplete) list of Canonical Orthodox Christian jurisdictions and their Primates that can be found in the USA. All of them are in communion with all of the others:
Greek Orthodox Archdiocese (an Archbishop under the Patriarch of Constantinople in Istanbul, Turkey).
Serbian Orthodox (a Metropolitan under the Patriarch of Serbia)
Antiochian Orthodox (Patriarch of Antioch)
Romanian Orthodox (a Bishop–I think–under the Patriarch of Romania)
Patriarchate of Jerusalem (Patriarch of Jerusalem, there may or may not be a Bishop in America, I’m not sure)
Russian Orthodox Church (only a single Cathedral–under the Patriarch of Moscow)
Bulgarian Orthodox Church (Synod of Bishops under the Patriarch of Bulgaria, I think)
Antiochian Orthodox Archdiocese (Autonomous under an Archbishop who is overseen by the Patriarch of Antioch)
Then two groups exist whose canonicity tends to be stronger the further away you are from the other of the pair:
Orthodox Church in America (Metropolitan of All America and Canada). In communion with Moscow and the Greek Archdiocese, among others.
Russian Orthodox Church Outside Russia (First Hierarch of their Synod). In communion with Serbia and Jerusalem.
Now, to confuse matter, quite a few Romanian Orthodox are in the Orthodox Church of America (preserving their unique practices therein). There is a Carpatho-Russyn diocese under the Patariarch of Constantinople, and I don’t want to even think about the Ukrainians. Then there are the odd assorted Poles, Czechs, Cypriots, and others who generally filter into other groups, and ROCOR is in communion with “Old Calendarist” Greek groups that would rather eat radioactive glass than be in communion with the Greek Archdiocese, even though the Greek Archdiocese is in communion with Serbia, which is in communion with ROCOR, etc.
Okay, so how did we get this scandalous mess?
Short answer: Bolsheviks.
Longer answer: Before the Bolshevik coup in Russia, it had been fairly well established (with some grumbling), that the Russian Orthodox Church had precedence in the Americas, or at least in North America. The oldest known active Orthodox missionary work in the Americas was done by St. Herman, Apostle to the Americas. St. Herman was part of the Orthodox Russian group. By Orthodox practice, in obedience to Scripture, Russia had laid the foundations, so other jurisdictions were not to build on them.
So, for a while, at least in theory, all Orthodox parishes in the Americas were under Moscow. However, some allowances were made to permit Greeks, Bulgarians, Serbs, Rumanians, etc. to have their own language and practices. For the Russians, this wasn’t as big a deal as it might seem. They had a long history of translating Orthodoxy into other languages as they conquered Siberia, and the native Japanese and Chinese Orthodox communities are ultimately of Russian origin.
However, as was to be expected, there were tensions. Greeks didn’t like having to be under an “inferior subculture”. Rumanians, Czechs, Serbs, didn’t like being under Russia for historial reasons. The Arabs were often left out in the cold, altogether. Nevertheless, for all its problems, some form of unity was getting knocked together (hopefully more often than heads were knocked together). Sufficient converts were entering Orthodoxy in America that English-language liturgies were being explored.
And then the Bolsheviks. At first, the Bolsheviks told a pleasant lie–they would permit the Church to exercise its functions without state interference. As soon as it became plain this would not be the case, Patriarch St. Tikhon issued first an ukase calling for the defense of the Church in Russia against the Bolsheviks. In 1920, St. Tikhon issued an ukase telling Russian hierarchs and those Orthodox under Russia to organize under whatever Orthodox authority could protect them, as Moscow could no longer act independently.
So, the Greeks went with Athens, who placed them under Constantinople in return for Constantinople’s recognition of Athens as autocephalous. The Arabs went with Antioch, in essence, everybody “looked home”. This left the Russians in a real lurch. Some looked towards Constantinople and were placed under a specifically Russian Episcopate. Others remained tied to Moscow. Others determined that the ukase gave them effective authorization to form a synod wherever they might be and soldier on as a Church in exile.
The Russians in the USA who remained tied to Moscow were mostly immigrants. The Russians who formed the new synod were mostly refugees from Bolshevism. They formed ROCOR In 1970, the Russians who had remained with Moscow were granted autocephaly and became the OCA.
It should be noted that the Greeks underwent a very similar schism during the overthrow of the Greek monarchy. However, since Greece’s new government did not make it a habit of becoming mortal enemies of the USA and trying to shut down the Church in general, matters were easier to patch up (mostly).
So, around 1920, all these groups went off their separate ways. The following things have kept them apart since:
Inertia. Probably the number one cause.
The Patriarch of Constantinople. An odd thing to claim (especially since he is my Patriarch), but here is my reasoning: BARTHOLOMEW holds the opinion that he is automatically the rightful Patriarch of all “barbarian lands” (any territory outside the old Roman Empire) and of all “the Greek diaspora”. However, the claim is made on fairly shaky documentary grounds. Nevertheless, it is an impediment to unity in the USA. The only unity that BARTHOLOMEW seems to wish is unity under him. Perhaps he is correct in his reasons. Perhaps he is not.
The Russian Revolution. Like it or not, some people are still fighting the Russian Revolution. However, this matter is beginning to resolve amicably as those who collaborated with the Soviets and those who suffered under them get old enough to move on. There have been friendly talks between ROCOR and Moscow, but they are still at the very polite level. However, ROCOR still refuses to recognize the canonicity of OCA, and vice-versa.
Many of the other Orthodox jurisdictions no doubt do not want to be “swallowed” by the larger jurisdictions, and this will also lead so resistance to integration.
There are some red herrings that get waved about from time to time, but these issues are invariably charicatures.
