We are called Roman Catholics because St. Peter was the first Bishop of Rome, and all the Popes who have come after him have been Bishops of Rome also. The word "Roman" does not really add anything to the title of the Catholic Church, because the Church founded by Christ with St. Peter as head, is the Catholic Church. So all Roman Catholics are Catholics, and all Catholics are Roman Catholics.

However, about a hundred plus years ago, some Protestants, members of the Church of England, started calling themselves Catholics, and the title "Roman Catholics" was used by them to mean Catholics in agreement with the Pope. As you know from history, they had no right to the title of Catholic. So real Catholics do not like the use of the title "Roman Catholics", as if there were other kinds of Catholics.