Why Hymns Are at the Bottom of the List

Even under the current General Instruction of the Roman Missal, hymns are listed only as substitutions for the chant Propers in the Roman graduale; indeed, using a hymn for the Entrance, Offertory, and Communion is the fourth option. Why?

Basically, there is an incompatibility between the form of Scripture and the form of hymns. Hymns and songs are metrical and symmetrical; they have a regular rhythm and lines of the same length. Scripture is not usually metrical and symmetrical. That means that Scripture has to be edited, interpreted, or rephrased to fit into verses with a set number of measures. Doing so without changing the meaning of Scripture requires a composer who is both a poetic genius and well-formed biblical scholar.
— Mary Ann Carr Wilson 

The singing at this time is done either alternately by the choir and the people or in a similar way by the cantor and the people, or entirely by the people, or by the choir alone. In the dioceses of the United States of America there are four options for the Entrance Chant:

(1) the antiphon from The Roman Missal or the Psalm from the Roman Gradual as set to music there or in another musical setting;

(2) the seasonal antiphon and Psalm of the Simple Gradual;

(3) a song from another collection of psalms and antiphons, approved by the Conference of Bishops or the diocesan Bishop, including psalms arranged in responsorial or metrical forms;

(4) a suitable liturgical song similarly approved by the Conference of Bishops or the diocesan Bishop.[55]

If there is no singing at the entrance, the antiphon in the Missal is recited either by the faithful, or by some of them, or by a lector; otherwise, it is recited by the priest himself, who may even adapt it as an introductory explanation (cf. no. 31).

- GIRM #48