For today's Feast of the Immaculate Conception, Mark puts a substantial excerpt out there about how the Eastern Church views Mary's Immaculate Conception. Do go read it. If you like it, I can guarantee you are going to like the books he wrote. Here's a bit to get you started.
That said, the question still remains: If the Immaculate Conception is truly apostolic teaching, then why do the Eastern Orthodox Churches reject it? After all, those Churches trace their lineage to apostolic times just as the Catholic Church does. To answer that, we have to understand why the Roman Church developed her doctrine in the way she did and why the East did not take the same path.
Some people have the notion the Eastern Orthodox Churches reject the Immaculate Conception because a few early Eastern Fathers (Origen, Basil, and John Chrysostom) expressed a couple of doubts about Mary's sinlessness. Origen thought that, during Christ's Passion, the sword that pierced Mary's soul was disbelief. Basil had the same notion. And John Chrysostom thought her guilty of ambition and pushiness in Matthew 12:46 (an incident we have already examined).
But the remarkable thing about these opinions is how isolated they turn out to be. Essentially, they demonstrate (once again) something about the development of doctrine that we've already seen in connection with the Trinity: The Catholic Church is not a monolith and her people, even very good people, sometimes voice in good faith ideas that end up departing from the orthodox norm. For the reality is that, apart from these three, the overwhelming consensus of the Fathers in both east and west is that Mary is "most pure," “formed without any stain,” "all-Holy," “undefiled,” "spotless," "immaculate of the immaculate," “inviolate and free from every stain of sin,” and created in a condition more sublime and glorious than all other natures.