According to this abbreviated
history of the Mass, when low Masses became common, the priest would say in a low voice at the low Mass those parts that others would have said or sung at a high Mass. And later, priests began saying those same prayers in a low voice at high Masses, too.
So it was sort of a historical accident, each step along the way a consequence or development of what preceded it, without the whole thing ever being designed from scratch. There was probably a practical aspect, too, to keeping most of the Mass fairly quiet, in times when there might be multiple Masses being said at the same time in a church. More peaceful, less distracting. (You can still see side altars in many older churches).
And I

St. Francis de Sales as well. A great saint!