You are generalizing. That is not necessarily true in every part of the U.S.
There was a pretty wide gulf between urban and rural areas when it came to FM. And some urban areas were slower to develop it than others.
Dondd didn't specify the years he was talking about, which makes answering this point harder. In northern Missouri and southern Iowa, there was
no
reliable FM activity until there was a burst of it from 1966 to 1968. The first FM station I heard as a pre-teen living near the Iowa-Missouri border at the time, was KRXL from Kirksville, Mo., which arrived in 1968. (KRXL is still around and never changed its call letters!) It was the first reliable FM signal on the Iowa-Missouri border. Otherwise, on good days, there was WHO-FM with beautiful music/pop classical and superpowered KDMI-FM, with paid religious programming. The first Top 40 station on FM in the area was KGRC from Hannibal, around the start of 1969.
Columbia got its first commercial FM late in 1967 with KTGR-FM, simulcasting its daytimer AM, also a Top 40 station with some album rock at night. No stereo, by the way. The first FM station there was a non-commercial 10-watter that wasn't actually on much.
There was more activity in the large Missouri metros earlier. But the mid-1950s were a dismal time for FM no matter where you were. Kansas City was down to one FM, KCMO-FM, for several years.
The move to the Iowa-Missouri border was a big comedown from living in Albuquerque. But a story from Albuquerque. One day I had to stay home from school sick and my mother got a friend to come to the house to watch over me. She brought along a radio to listen to classical music. Already I was interested in radio, and I saw that it had different numbers on it. "What's that?," I asked. She replied, "Oh, it's an FM radio. It only gets KHFM." Now I realized that was odd, since there were a few other FMs on at the time...KANW for the schools, KARA-FM for beautiful music, KDEF-FM, plus KRST coming on the air during that time, though KOB hadn't yet joined the FM crowd and KGGM never did.