I believe the overarching reason why the Orthodox (Greek, Coptic, Serbian, Russian, etc.) Church has remained THE TRUE-to-Christianity's-Original-Form Church is easy to figure out--particularly for the many Protestants who do not really HAVE any HISTORY OR TRADITION FROM their fellowships, most of which are Evangelicals off-shoots from main stream Protestant denominations with a more traditional Christian base like Episcopalian/Angligan or Lutheran--and plainly obvious for those who dare to research questioning the status quo of their Church.
But before I give away the simple-for-anyone-to-discover reason which is somewhat obvious to most Christians, who know some Church history, that the Orthodox Church is indeed the oldest (and we could go back to the Ethiopian eunuch with Philip, or Antioch, if you'd like), most original and truest Church, I will say the following: Martin Luther (see footnote below for source) did not want the Catholic Church to be splintered into into the thousands upon thousands of Protestant churches that exist today who all believe, teach, preach and understand Christianity DIFFERENTLY, whereas the Orthodox Church pretty much has remained the same Church from day one. Luther wanted the Church reformed, but he agreed with the originality of the Orthodox Church and their stances against the Catholic Church which had practices of clerical celibacy, papal supremacy, purgatory, indulgences, and Communion by bread alone.
So my reason : to really understand the Orthodox Church and its true originality you must understand that it is a Church with a goal, still to this day, of keeping to Church/Empire unity for the sake of unity all based on the First Ecumenical Council in Jerusalem in the Book of Acts, 15 where it says "22 Τότε ἔδοξεν τοῖς ἀποστόλοις καὶ τοῖς πρεσβυτέροις σὺν ὅλη τῇ ἐκκλησίᾳ ..., Then it pleased the Apostles and the Presbyters and the whole Church...."
In other words, the Orthodox Church, believes, that from day one, The Catholic (meaning universal in Greek, Καθολικός meaning according to the whole implying Universal, East and West)-- which is what it actually was called and the Creed states this too--Church was and still is to stick to a conciliar system of agreement amongst clergy, hierarchy, laity and East and West , the very system that West decided to go against for its own pursuits starting from around the 6th Century with the great heresy of the supposed infallibility and primacy (amongst all bishops of the Empire) of the Bishop of Rome, the Pope. This, of course, is the main cause of the Schism of 1054.
16th Century Lutheran & Orthodox Exchange