Unpleasant music analysed

Why is so much 20th century music still considered "difficult" if not downright unpleasant, and especially church music?

I was looking again at a mass setting that we have been practising, that left me literally with a sour taste in the mouth. There were in fact, two particular ingredients. It was in the vernacular but I will refer to the parts by their Latin names. One was the rhythm of the Kyrie - a jazzy der-dit-di-der-dit-di-der-dit-di-der. A real get-you-moving rhythm. To make matters worse, the tune was not in a key, nor was it in a recognisable mode, though it seemed to be moving towards Gregorian mode 4 - the most difficult and unsettling one, being most removed from a major scale. All exactly wrong for the start of the mass, I would suggest. There seems to be a lack of sense of what is appropriate.

Then there was the Agnus Dei, which goes

1 Agnus Dei, qui tollis peccata mundi, miserere nobis
2 Agnus Dei, qui tollis peccata mundi, miserere nobis
3 Agnus Dei, qui tollis peccata mundi, dona nobis pacem.

In this setting, (1) and (3) were fine - nice and harmonious. (2) had a run of sour discords in the "qui tollis peccata mundi". I suppose this was intended to create a sense of tension. There is nothing wrong with discords, used judiciously. Bach used them, and so did a number of seventeenth century composers. But judiciousness is key, and the Agnus Dei is not the right place in the mass to play this game. One wonders what the composer was trying to achieve as he had in fact written a perfectly good setting with first and third lines, since line (2) can be a repeat of line (1).

The interesting thing is that if one finds music unpleasant, there are specific reasons why.