(I haven't done exhaustive research on this, so someone please correct me if I am wrong.)
Papal encyclicals, up until the time of Pope John XXIII (the convener of the Second Vatican Council) were addressed to the clergy of the Church, and usually specifically the Archbishops and Bishops of the Church (whose job it would be to disseminate to his flock the information contained in the Pope's missives).
However, with Pope John XXIII's last encyclical, Pacem in terris, the "tone of voice" of the Papacy changed. Here is how John addressed his encyclical on how "Peace on earth, which all men of every era have most eagerly yearned for, can be firmly established only if the order laid down by God be dutifully observed": "To our Venerable Brothers the Patriarchs, Primates, Archbishops, Bishops and other local Ordinaries in Peace and Communion with the Apostolic See; to the Clergy and Faithful of the whole world; and to all men of good will."
Not only did the Pope address the whole Church, he addressed "all men of good will".
Pope Paul VI followed this example in his first encyclical, Ecclesiam Suam, which he addressed to the whole Church "and to all men of good will". And Pope John Paul II did the same thing which his first encyclical, Redemptor Hominis, addressed to the whole Church and "to all men and women of good will".
I think the reason for this change in the "tone of voice" is because of the need for increased vigor in gathering all humanity into the single Church founded by Jesus Christ. These three encyclicals which inaugurated this tone of voice were about peach on earth, Christ's Church, and Christ as the Redeemer of each and every man. These are universal calls, because the Catholic Church is and must be universal. These are universal matters for each and every man.
Now, Pope Benedict's first two encyclicals were not addressed in this way. Before you leap upon this, permit me to explain.
Pope Benedict's encyclicals, Deus caritas est and Spe salvi, deal with love (charity) and hope, two of the three theological virtues (the third being faith). He is addressing the Church on these important virtues so that those who are already in the Church can be recentered on Jesus Christ and the virtues which the Holy Spirit infuses us with so that we can continue this external mission of ours, out in the world. He is not cutting off communication with the world, but is addressing those of us whose mission is in the world.