First of all, FM didn't reach audience parity with AM until about 1982, not "the 70s". There were TONS of AM music stations in the 80s, and in fact there still are hundreds of successful AM music stations around the country. There isn't one single reason why much of the audience left AM music stations for FM, but I can name MANY instances where, as FM started to equal AM's audience, AM rock stations started changing formats WILDLY...first trying Adult Contemporary (killing off young listeners), then Adult Standards (killing off listeners in their 30s and 40s), finally switching to talk. In effect, in MANY cases, stations that had ratings THEY WOULD KILL FOR NOW with all the fragmentation that's set in LEFT THEIR LISTENERS, rather than the listeners leaving them. I think successful AM music stations largely committed mass suicide in the 80s. Stations around here that didn't "drink the Kool Aid" in the 80s, that continued playing music, are STILL DOING WELL more then two decades later! They didn't desert the audience, so guess what...IT STAYED! Take Wilkes County where I live. EVERY FM MUSIC STATION from Greensbor and Charlotte is audible here, but the number one station year after year is WKBC AM, a country music station that NEVER CHANGED FORMATS!
Again, this is what happens when people who don't work in radio pontificate about the industry, as if their knowledge was not only fact, but universally known by everyone except for those poor, uninformed souls WHO ACTUALLY WORK IN RADIO! At any rate, AM stations DID largely adopt talk formats. But that wasn't exactly the end of 'em! And there are many AMs that can and will do better with music formats in the future thanks to HD. By the way, people who don't know their rectal orifices from a hole in the ground frequently say things like "AM Stereo was invented to save AM in the 80s, and that didn't work"...not realizing that AM stereo was actually proposed (and worked just fine) in the early 60s, when AM had more than 90 percent of the audience, but the FCC decided to put it on hold to give FM an advantage...because FM was FAILING MISERABLY!
Anyone who thinks there's no new music on the radio obviously hasn't checked past the commercial outlets owned by Clear Channel, CBS, etc. to the public and college stations which offer TONS of music you can't hear anywhere else. Check out a couple at WNCW http://www.wncw.org
or WXPN whose "Exponential Radio" has a phenomenal variety of music, and is carried around here on the HD2 channel of WFAE Charlotte http://www.wxpn.org
By the way...programmers are right...people ARE more comfortable with musical that resembles things they've heard before, just as mystery readers are comfortable with the genre when it contains elements from most successful mysteries. That doesn't mean that an occasional "what the f@@@" moment isn't both unexpected, but delightful...and good programmers know that. Music, like all of culture, is never something entirely unlike what came before. It begins with a foundation laid by previous generations, as does all of life! We ALL, whether we admit it or not, stop when we hear something familiar...not necessarily an old song, but one with familiar elements. We're comfortable with what we already like. SO WHAT? Many Joni Mitchell fans started with "Both Sides Now", but stayed for "Hejira", and "The Hissing of Summer Lawns"...FAR LESS CONVENTIONAL FARE! Familiar is the "hook", NEW is the vitality that keeps things exciting. And PLENTY of radio these days offers both, though admittedly not enough. Which is why multicasting on HD offers a GREAT opportunity to expand variety!
And "commercials" killing radio, or tv, is your FANTASY. With a bazillion channels available on the internet, and satellite, more than 250 million Americans listen to terrestrial radio every week. With HBO, Showtime, Starz, Cinemax, and a few hundred other commercial-free channels now decades old, the big commercial networks still OWN PRIMETIME! There are two choices...the listener pays for the programming, or advertisers do. The model that has succeeded and thrived generation after generation (15 million to 250 million in terms of satellite vs. terrestrial) is ADVERTISER SUPPORT!
Recent studies have shown that most listeners DO NOT tune out from terrestrial stations when commercial breaks come on. After a 3 minute break, between 80 and 90 percent ARE STILL LISTENING! So much for theories about how radio works from people who aren't in radio. Want to try some amateur neuro-surgery?