The authors speculate that the melodic changes that occurred in 1975 could represent the rise of genres such as new wave, disco and stadium rock. Those occurring in 1996 and 2000 could represent the rise of hip-hop or the adoption of digital audio workstations, which enabled the repeated playing of audio loops, they add.
The authors note that although the complexity of popular melodies appears to have decreased in recent decades, this does not suggest that the complexity of other musical components—such as the quality or combinations of sounds—has also decreased.
They speculate that decreases in melodic complexity could result from increases in the complexity of other musical elements, such as an increase in the average number of notes played per second, to prevent music from sounding overwhelming to listeners. Additionally, they propose that expansions in the availability of digital instruments may enable musical complexity to be expressed through sound quality, rather than melody.
I don't know much about music, so I thought of posting it here rather than in the ~science territory. More knowledgable people might be able to comment on these observations.
Does it match your experiences?