Ever since the Telecommunications Act of 1996, commercial radio stations have become heavily formatted and playlists shrank until some stations had a playlist of only about 120 songs. Recent formats have relaxed a bit but playlists only grew about threefold. Of all the great music that has been released in the past 40 years or so, limiting that output to, at most, 500 songs, and it becomes clear why commercial rock radio has had its problems.
In 2008, Clear Channel's stock was trading at $18.50 a share and Cumulus was trading at around $20. Today, (December 2016) Clear Channel (IHRT) is trading at $1.39 and Cumulus (CMLS) at $1.10 Clear Channel owns 850 stations in the US and Cumulus owns 570. Clearly the two largest players in the market are struggling.