It's actually pretty simple. Making up every song ever made are chord sequences and melodies that can be broken down and analysed with music theory. Through the Sixties and Seventies, we had a revolution of pop/rock music that brought new ideas and new chord structures to music. You had bands like Zeppelin, King Crimsons, Yes, the Beatles, Jean Michel-Jarre etc, reaching out and bringing in new ideas, new ways to play melodies, new ways to structure songs. Outside of Pop, you had artists like Miles Davis, John Coltrane and Mahavishnu Orchestra literally revolutionising not just song structure, but the very structure of music.
Ever since the Eighties, mainstream music has been on a downward slide. Instead of trying to make new advances in music, the way Miles Davis and the Beatles managed to, we've simply dressed up the same chord sequences under new toys and gadgets, and called that progress. The fact that the majority of the biggest pop hits of the last few years (Gnarls Barkley: Crazy, GOTYE: Someone I Used To Know, Adele: Rolling In The Deep, etc) all revolve around the same Minor I-b7-b6 chord sequence should be an indicator of how stagnant pop music has become. That chord sequence was already old hat when Phil Collins used it for In The Air Tonight. The likes of Zeppelin and Hendrix had already done everything that needed to be done with that chord sequence in Stairway and All Along The Watchtower, respectively.
If we do not identify stagnation and repetition when it occurs, then how can we ever expect music to innovate and take us to new places. The music of the Fifties and Sixties was powerful enough to ignite the biggest social revolution the West has ever seen. Shouldn't we strive to create music of that power and importance today?