Jesus’ promise of protection for His Church rests with the Pope. The problem is that the Orthodox Churches broke off from the Pope (over several disagreements, including the filoque and politics (Rome was a backwater, Constantinople was huge, glorious, and politically powerful, so why take orders?)).
When other groups have been separated against their will (like the Chaldeans) or have separated with the Orthodox and come back (like the various Byzantine rites), they have accepted what has been settled during the time of their absence. They don’t get to say, “Well, we weren’t at that council, so we can believe something different.”
Plus, God gives us Truth as we can handle it. As Jesus pointed out to his disciples in the Gospels, there is more truth than what is written down in the Gospels, but they weren’t ready for it yet. As the Church considers and prays over certain things, we come to better understandings. Gradually, some of the “we don’t know” spots are filled in with clearer statements of doctrine.
This is why the Marian doctrines were defined so “late.” By the 300’s, the Church was still trying to understand Jesus as both God and Man, what the Trinity is, etc. As essentials were better understood and explained (and society became settled enough for monasteries and other centers of learning), later theologians looked at how those essentials played off each other, studied Scripture further, and began to see more details in the theological picture.