If you're a faithful Catholic, then when you're at the Mass you are in the actual physical presence of Christ.
It's not that one shouldn't have fun in church (we have a wonderful time singing beautiful music) but that one should be reverent in the presence of God. And a lot of the schlock pop stuff just isn't reverent - it's noisy, and it diverts attention from the Holy Sacrifice to the performers. And as commercially-based music, it brings the values of the marketplace into church. While a lot of folks support this because it "brings in the young people," I've found it may bring them in temporarily, but they don't stay. On the other hand, they flock to the traditional Masses, and they seem to keep coming.
Benedict XVI explains it a lot better in his book, The Spirit of the Liturgy.
Actually I understand completely... I'm just bein' a smart aleck. I grew up in a pretty quiet and traditional Lutheran church, just an organist and a blue-haired choir to help keep the shy and quiet congregation on the right tune in the hymnal. I've been to a few of the rockin' and rollin' new churches with the loud music and the power point presentations and found it a bit um.... TV. But man, they sure felt saved.
I think the appeal of the Catholic Church is it's traditions. I can see the value in maintaining that, really.