>>Why do you think the Liber Usualis has different chant settings for every day of the liturgical year? <<
And why do you think we only use the common chants at Holy Mass? Just because they are written doesn’t mean they are used, eventhough they are allowed.
Our chants don’t change for our Latin Vernacular nor our Traditional Latin Mass.
They don’t change in our vernacular Holy Mass both English and Slovak.
Perhaps choir directors like liturgical committees perpetuate their jobs by innovations.
My point was that the Church from old times has recognized the need for some variation in the music (as opposed to the text, which does not change). That's because the folks compiling the Liber were musicians and understood what music and musicians require in order to do their best.
We do have three Mass settings - one for the Latin Mass and two for the Ordinary Form. The congregation doesn't seem to mind (although they are struggling with the Latin).