"When a space becomes available, work emerges to fill it."
David Byrne published a longer version of the talk he did at TED recently on his blog. The video of his talk is not yet up, but I highly recommend reading about the evolution of spaces and the music made for them from African tribal to Gregorian chants from cathedrals to the super-tricked out car stereo.
Western music in the medieval era — Gregorian chants and such — was performed in these stone-walled Gothic cathedrals, and in monasteries and cloisters, which had somewhat similar acoustics. The reverberation time in those spaces is very long — more than four seconds in most cases — so a notes hang in the air and become part of the present. Shifting keys would inevitably invite dissonances and a sonic pileup, as the previously sung or played notes would overlap with the new ones. So, what evolved is what sounds best — it is modal in structure. This music often uses very long notes and slowly evolving melodies with no key changes whatsoever. It works beautifully in these spaces — in fact, the space even improves the music… it gives it an otherworldly ambience. It’s often assumed that this music was harmonically “simple” (having few key changes) because these composers hadn’t “progressed” to complex harmonies yet. I’d argue that in this context there would be no need or desire to include complex harmonies, as it would have sounded horrible. Creatively they did exactly the right thing.
Here’s the church where Bach did a lot of his playing and writing in the early 1700s:
As you can imagine, there was already an organ there and the sound would be less reverberant than in the Gothic cathedrals, but still echoey. The reverb was so much less that, famously, Bach and others could introduce key changes into their music.