From Burkholder et al.’s History of Western Music, 9th edition (pp. 938-939):

The demands on performers by composers like Berio and Carter [and] Babbitt, Stockhausen, and Boulez, were matched by their demands on listeners. Each piece was difficult to understand in its own right, using a novel musical language even more distant from the staples of the concert repertoire than earlier modernist music had been. Compounding listeners’ difficulties was that each composer and often each piece used a unique approach, so that even after getting to know one such work, encountering the next one could be like starting from scratch.