Much of the discussion in this thread has centered around cities. Metro markets. And that is where the big dollars are.
But in the mid 60s, something began to happen in Midwestern Rural areas. (In Southern states because of low ground conductivity, every little Mayberry could have it's own AM station... maybe two.) But up in the cornbelt where AM signals just go on and on and on, there were some county seat towns of 12,000 to 18,000 that had NO radio station. Towns that were prosperous not only because of corn belt farming, but industrial factories making car parts, furniture and who knows what all. With no AM frequencies available, some gutsy, creative entrepreneurs began building stand-alone FM stations... not with the traditional easy-listening music found on so many city stations, but with high school sports and trading post and local news. All those stations added together may now have amounted to much in the big scheme of radio as a whole, but they began pitching their stations at the ad agencies in big cities and state capitols, and CocaCola bottlers began bugging national agencies to put some of their ad money into local high school sports on <gasp!> an FM only station in rural America. And some of the owners of these little tea-pot FMS has worked in larger cities and they had an in with some of the account reps in the big city ad agencies.
Again, this rural adoption of FM as REAL RADIO raised the awareness of the industry and the advertising industry and gave everyone in the industry the inspiration to begin thinking bigger about the potential of using FM radio as REAL RADIO.