We live in an age where music is more diverse than ever but the radio is at its most homogenous in decades. Why is this?
One possible answer we are more homogenized as a culture. Maybe we want radio to be this way.
A more likely answer is that corporate ownership of radio stations by a single entity has limited what you hear on the radio.
Power to play new things on the radio used to be distributed between dozens of DJs in different local markets. Dewey Phillips, a Memphis DJ, played the first Elvis record in Elvis’s hometown. Other examples abound. (wikipedia)
Today, the playlist for hundreds of local radio stations is determined in a central office by a company named iHeartRadio (formerly Clear Channel). You get your music on the radio by getting a meeting with these folks. These meetings are available only to the powerful, and very little music bubbles up from local scenes to become radio hits anymore.
Source: Distributed DJs
Top 40 music today is mainly a process of Swedes and Divas.