Its arguable that the decline in popularity of rock music over the past decade has forced Hot AC to transition and include more rhythmic based product as well. There is an American study that shows the decline in the prevalence of rock music in America between 2000 and 2010, but I can't find it.
Here is a similar story about the top 100 hits in the UK last year, though:
http://www.bbc.co.uk/news/entertainment-arts-12151129
It can be said that rock as a singles-driven business in the age of iTunes doesn't succeed in the same way as when the album was king; combined with being in a PPM era when stations are rewarded for running catchy pop songs into the ground at the speed of 170 spins a week. It lends itself to a shift in formats where rhythmic product is not only on top at CHR but making in-roads at Hot AC that would have been unimaginable ten years ago.
Here is a similar story about the top 100 hits in the UK last year, though:
http://www.bbc.co.uk/news/entertainment-arts-12151129
It can be said that rock as a singles-driven business in the age of iTunes doesn't succeed in the same way as when the album was king; combined with being in a PPM era when stations are rewarded for running catchy pop songs into the ground at the speed of 170 spins a week. It lends itself to a shift in formats where rhythmic product is not only on top at CHR but making in-roads at Hot AC that would have been unimaginable ten years ago.