Because there is only one "Western rite" in the sense of a Tradition or a liturgical-spiritual-theological-doctrinal-disciplinary patrimony. Which is, of course, an accident of history. But for the collapse of the Western Empire, the Great Church of Africa might have become a second Western Patriarchate, with its own unique theological and liturgical perspectives (possibly a highly semitic one, given the Punic background of the population). Gaul might have become a third Western patriarchate, possibly encompassing both Spain and Britain. Anyway, it didn't work out. One can speak of a Latin or Western rite, because there is only one Western Tradition; one cannot speak of a single Eastern rite, because there are several distinct Eastern Christian Traditions (Byzantine, Coptic, East and West Syrian, Armenian). If I say "Eastern rite", to which of these does it refer? Yet if I speak as if all were one, I do a disservice to the uniqueness of each.
