Well, I don't speak for OOxy, obviously, but we haven't had an undisputedly Ecumenical Council since Nicea II, so we're in a similar boat. Compare the Roman Catholic Church, which has one every couple of hundred years because there are too many people in the Church and they need to alienate some. (I'm looking at you, Vatican II...and maybe even Vatican I).
Typically, in the Eastern Orthodox Church, and I'm guessing the Orientals have the same conception, Ecumenical Councils are called to deal with major heresies that arise. Maybe no major heresy has arisen within these Churches since they shut down Nestorianism?