I cannot speak for radio employers/employees, but I'd say it's a case of more music being downloaded nowadays, iPods/iPads, basically folk getting their music anywhere they want, without having to wait to hear it on the radio (waiting through commercials and such). Even the Billboard charts don't rely on radio near as much as it once did.
Add to that the fees that the music industry may be imposing on the stations, to a point that they feel it's not worth the hassle, especially if less people are listening to radio for music, anyway.
Music on AM died pretty much 25-30 years ago, to the point where talk dominated. And in 2012, people want their talk & news on FM instead of AM; AM is severely struggling. Now, FM has to innovate, or else *it* will become like AM---so the formats (sports/sports talk, for example) that were once AM-only have moved (or are moving) to FM.
Give FM 5 years---and see how the landscape will change even more!
(Am I about right here?)
cd