All that ^, and because the Roman Catholic Church does not use the Divine Liturgies of St. John Chrysostom and St. Basil the Great.
The Roman Catholic Mass does include the Nicene Creed as a "theological statement of faith in the dogma of the Incarnation" which is what "Only Begotten Son" is, I believe.
Is the hymn used in the Orthodox Western Rite liturgies? If not, why not?