I’m not sure how he chose his various composers, but imagine that he familiarised himself with their talents and chose whichever he thought best suited to the film in hand. Generally the music is pretty sparse, and has to be considered as just one element of the total soundscape, which he seems to have managed as carefully as his camera angles.
True, but there’s actually a considerable amount of music in many of his films, especially in all the Giovanni Fusco collaborations. And most of it is rather tonal and accessible. Then he took a left turn now and then with the sound designy Gaslini score for LA NOTTE or the pop/rock/jazz music in films like BLOW UP, IDENTIFICAZIONE DI UNA DONA, ZABRISKIE POINT etc. If you don’t already own it, this album is recommended:
