There are a lot of reasons for the demise of radio...but it definitely started with the Telecommunications Act passed in 1996. If you're unfamiliar, read up on it. It damaged radio permanently. Clear Channel benefited from it and ruined a lot of great radio stations as a result. Problem is, advertisers continue to pull out of radio ads, which forces stations with thin margins to cut budgets and allow for cheaper salaries and mediocre personalities.
As a result of that, people have found and continue to find alternatives, like CDs, podcasts and mp3 players.
Radio will die when the buck stops and the buck keeps getting smaller. It's just a matter of time and radio will be dead.