The reason is very simple: Meter.
Meter is the regular beat of a tune. It is present in all songs with a regular beat. We notice this more in modern rock music because it is emphasized with drumbeats and guitar strumming, but it is also present in hymnody - though not as pronounced. This regular, pulsating rhythm appeals to our lower appetites - lower because it is base. I’m not saying it’s bad - I LOVE to listen to catchy tunes on the radio as I drive. It keeps me alert and makes my drive enjoyable. I often tap my hands on the steering wheel and pretend I’m the drummer (and get funny looks from passers-by).
However, at Mass we can’t let these instincts overtake us. Our faith and love as Christians are primarily acts of the will, not feelings or emotions. We choose to love, have faith, be contrite, etc… Not that emotions are bad, but they can be misleading. Example: you confess your sins in the confessional with contrition and a resolution to amend your life. You walk out of the confessional but still feel guilty for these sins. This “feeling” of guilt should be rejected by your will because you trust in Christ’s promise of forgiveness. So, your feelings must always be subjected to your will. If your mind tells you that it is improper and distracting to dance at Mass, then you should obey your will, not your feelings.
Incidentally, this is one reason why I prefer Gregorian chant. Many Western ears (like mine) are initially drawn to chant and repulsed at the same time. Why? It has no meter. Chant has what is called a tactus. It is not a regular beat, but a sort of pattern of pulses. Chant is like speaking while using notes. To Western ears, this irregularity seems both ethereal and estranging. The lack of a regular beat also is disconcerting for those who are used to regular beats. It takes getting used to, but the wisdom of the Church Fathers is evident when you get acclimated to it.
