A good deal of the problem lies in the lack of quality Catholic liturgical music and a problem that I call “Liturgical Illiteracy”. The Church certainly values sacred music as we have a treasury of hymns that goes back 2,000 years. However, she has her standards as to what can and cannot be sung during the Mass.
Unfortunately, what shows up in song books published by OCP and, to a certain extent, GIA, is not suitable for Mass. It is as though the publishing houses have never heard of Musicam Sacram and Liturigam Authenticam, two of the authoritative liturgical documents issued by the Holy See.
Something else to consider is the fact that what we do in the Holy Sacrifice of the Mass is vastly different from what Protestants do at their services. They have the Word, which consists only of biblical readings, a sermon and songs thrown in for good measure. However, we have both the Word (the Liturgy of the Word) and the Sacrifice (the Liturgy of the Eucharist). The Mass constitutes more than just singing; at ever Holy Sacrifice the veil between time and space is lifted and we are just as much present at Calvary as were the Blessed Mother and Sts. John and Mary Magdalene. Furthermore, we are drawn into God’s time and the veil between heaven and earth is lifted so that the entire Church, the Church Militant (us), the Church Triumphant (the saints in heaven) and the Church Suffering (the souls in purgatory) is present at every Mass. In other words, the Church does not consist of the warm bodies in the pews, it encompasses the saints and the souls in purgatory (the Communion of Saints).
I have yet to see any modern hymns that take this into account, let alone the Sacrificial aspect of the Mass.
It’s not just about the music. The Mass is more than that, so much more. However, genuinely sacred music, as the Church defines it, certainly adds to the solemnity and the dignity of the Holy Sacrifice of the Mass.
The faithful probably may not sing because the music is bad. I will not sing a song that I believe to be doctrinally and liturgically deficient. Neither will a priest friend of mine. Furthermore, I will not sing a part of the Mass that is paraphrased (the Gloria, the Sanctus and the Agnus Dei) because it is wrong. If the parts of the Mass are to be set into music, they must match the official text and not be paraphrased.
It is not that the faithful are not passionate; it’s that many are wising up to the deficiencies in the musical offerings of their parishes.