When man comes into contact with God, mere speech is not enough.
In his essay Music and Liturgy (p. 185 of The Essential Pope Benedict, originally from his book The Spirit of the Liturgy), Pope Benedict XVI names the standard by which all sacred music should be measured: Logos, or the Word of God that is Jesus. Some particular relationships exist between liturgical music and Logos:
1. WORDS - Christ sanctified human thought and speech by creating man in his image and likeness; as Aristotle puts it in the Politics, man is the only animal with the power of speech. Hence Benedict:
Thus the relation of liturgical music to Logos means, first of all, simply its relation to words. That is why singing in the liturgy has priority over instrumental music.
2. SOBER INEBRIATION - a term used first by St. Cyprian of Carthage, this is the opposite of drunkenness or intoxication from wine or, allegorically, worldly goods; rather, Cyprian is meditating on the cup of salvation from the Psalms, now revealed in Christian times as the Blood of Christ. For Benedict, this sober inebriation applied to music means:
...Music that draws senses into the spirit and so brings man to wholeness. It does not abolish the senses, but inserts them into the unity of this creature that is man...Not every kind of music can have a place in Christian worship. It has its standards, and that standard is the Logos.
So what kind of music DOES NOT meet the standard of the Logos?
On the one hand, there is pop music, which is certainly no longer supported by the people in the ancient sense. It is aimed at the phenomenon of the masses, is industrially produced, and ultimately has to be described as a CULT OF THE BANAL. "Rock", on the other hand, is the expression of elemental passions, and at rock festivals it assumes a cultic character, A FORM OF WORSHIP, IN FACT, IN OPPOSITION TO CHRISTIAN WORSHIP. People are, so to speak, released from themselves by the emotional shock of rhythm, noise, and special lighting effects. However, in the ecstasy of having all their defenses torn down, the participants sink, as it were, beneath the elemental force of the universe. The music of the Holy Spirit's SOBER INEBRIATION seems to have little chance when self has become a prison, the mind a shackle, and breaking out from both appears to be a true promise of redemption that can be tasted at least for a few moments. (caps my emphases)
Bluntly: pop and rock music are both inappropriate for Mass, pop music for its inherent banality, rock music for its inherent elementality, which presents an attack upon the dignity of the human person as a temple of the Holy Spirit.