"178. For vernacular celebration, the conferences of bishops may adapt the Latin hymns to suit the character of their own language and introduce fresh compositions,  provided these are in complete harmony with the spirit of the hour, season, or feast. Great care must be taken not to allow popular songs that have no artistic merit and are not in keeping with the dignity of the liturgy."
-General Instruction of the Liturgy of the Hours, chapter III.
"Because the hymns of the office are meant to be sung during the common praying of the hours, popular songs well known to Americans for morning and evening prayer have been incorporated."
-Instruction on the Hours, American Interim Breviary.
Because it seems like the bishops of the United States completely ignored a direction of the Vatican and decided to use popular songs anyway, and now all English-speaking countries have been forced to use popular songs rather than the actual hymns of the breviary*. And that obviously can't be true. I mean, would a bishop of the church ever ignore a liturgical directive based on his own preference and what he likes? Of course not!
*Hymns, which, by the way were written by saints and doctors of the church:St.Ambrose, St.Gregory, St.Augustine, St.Hillary, St.Bernard, St.Thomas Aquinas, St.Pius X, St.Prudentius,St.Robert Bellarmine, etc. Thanks guys! You threw out thousands of years of hymnody for songs that are at the moment, already old and outdated.