For countless generations, musical notes were laboriously constructed by hand. This limited the quantity of music humans could produce, but there were few humans, so it wasn't a problem.
Over time, as human population and therefore human music production increased, so did the construction of new notes, and the two growth rates roughly kept pace. However, in the 18th Century, music production exploded, and by the end of that century, the supply of the shorter notes, especially eighth and sixteenth notes, was almost exhausted. As a result, in the following century, music was typified by longer, slower sounds, or what we call the Romantic Movement.
Thanks to the Industrial Age, machine production of musical notes replaced manual craftmanship, resulting in a plentiful supply of notes by the beginning of the 20th Century. Sadly, the new machine-produced notes were of inferior quality. This resulted in Modern or Contemporary Music.
The huge growth of popular music in the 20th Century once again threatened to create an imbalance between the rates of production and consumption of notes. Fortunately, much 21st Century popular music doesn't seem to use musical notes at all.
