Here's an interesting filter to apply to this map: all of the innovative music from the last 100 years originated in the triangle between Memphis, New Orleans and Nashville—blues, jazz, R&B, rock and roll, and country. All of those genres were born in that small geographical area. It's the Silicon valley of modern music. You could also argue that hip hop is a permutation of blues/r&b.
What's also interesting is that now that triangle is "the established players" and the disruption comes from various other areas. Grunge from Seattle, Motown/R&B from Detroit, hip-hop from Atlanta, old-time from Asheville, electric blues from Chicago.
Much like how Apple from Silicon Valley owned the idea of buying digital music until Spotify swooped in from Sweden and disrupted that business model and forced Apple to change their plans, the Nashville sound is in constant danger of being disrupted from afar by similar-but-more-modern sounds.
