Surely there have been cultures fascinated with the more "dark" elements of humanity. Why is it that it was not until the 1950's and on that we started to see heavier genres of music come about?
Alternatively, why is there no "ancient heavy-metal" music, or anything similar, despite there being very dark literature (The Brothers Grimm, Dante's Inferno, to name a few) in the history of art?
I think there may be an issue with what your suggesting.
If you mean the vulgar, sexual, base, suggestive, and lude - these have been in music for as long back as we can find it. The allusions are just more subtle (as are most references in rock from even a decade ago).
Musically, rock isn't about a celebration of hedonism, necessarily. It's about the reduction of form, instrumentation, and themes to their most simple core function.
A lot of this has to do with the availability of recorded music (which allowed musical ideas to spread faster), the industrialization of instrument production (which made instruments cheaper to acquire and learn for amateurs), and the democratization of music from religion (Where it sat in the western tradition for about 1700 years) to the masses.
The other thing that helped, obviously, was amplification. It helped reduce ensembles from 10-15 players (Who you needed for volume in a dance hall) to three or four. With an amplified guitar, a single player could fill an entire room with sound.
But, there are dark themes and heavy tones in classical music.
Night on Bald Mountain, Mussorgsky, for example.
It's also important to remember the role that music plays in our life now. We can expect to hear thousands of songs in a day if we wanted to. Up until the 1800s, music was a real treat and part of a larger heritage of performance.
When instruments became cheap and recording made possible, musicians could spend more time expressing themselves because they didn't need to necessarily belong to an ensemble or serve a patron to thrive. This lead up to a larger focus on the performer over the performance.
It's hard to imagine, but up until the time of Beethoven, most musicians were seen as craftsman, not necessarily artists. Once the "Cult of the Artist" emerged, people were more free to focus on expressing themselves.
So, as you can see, there are a confluence of topics and complicated aspects to music that have made our current model possible. I hope that sheds some light on your question.
EDIT - Very happy to see my bachelors of music coming in handy :) If anyone has any other similar questions or I can elaborate, let me know.