Totally and completely arbitrary, m'dear.
Every culture's music development has been a thoroughly evolutionary process. That being said, I believe the basics of tonality were dictated primarily by the inherent characteristics of the musical instruments invented by each culture.
Ever hear someone try to play Western music on a sitar?
Someone correct me if I'm wrong, but, to my knowledge, only Western music has had a tradition of writing music. As a result, it has always been vastly easier to disseminate to other cultures, thereby making it the relatively dominant form in the world.
Here's a point to ponder: Wouldn't have been COOL if the ancient Egyptians had written down their music?
<small>[ 02-03-2006, 05:10 PM: Message edited by: OperaTenor ]</small>