Well, it is, FK, because it is a Roman Catholic dogma (as of 1854).
I'm not sure I understand why that is. I wonder if it has something to do with the concept of original sin
The western (Augustinian) concept of the "original sin," which you share with our Catholic brothers and sisters, is at the root for such a dogma, and one of the several theological disagreements the Orthodox share with the Church of the West, although -- paradoxically -- both sides of the Church believe Blessed Theotokos to be have been immaculate all her life, and venerate her as the Saint above all Saints.
Yes, and even though I have some idea of how close Orthodoxy and Catholicism are, I find it interesting that on some issues I find myself in more agreement with one, and on others the other.