Why is the Byzantine Catholic Church also called the Ruthenian Greek Catholic Church?

Why is the Byzantine Catholic Church also called the Ruthenian Greek Catholic Church? Fr. Hardon's *Catholic Dictionary* defines "Ruthenian Rite " as: >The liturgy used by the Ruthenian Catholics in Poland, the United States, and elsewhere. Said in the Slavonic language, it differs but slightly from the Byzantine Rite in the Orthodox Church. I've even seen such churches called Ukrainian.