Then there were those of us who were in suburban areas that grew into the null of the Top-40 station. In this case, St. Louis. St. Charles County, where I went to high school, was right in the null of KXOK's nighttime signal. You could get KXOK at night, but it was weak and noisy. Of course, KXOK was fine during the day, and we had daytimer KIRL in St. Charles, complete with sped-up records, but nighttime was prime time for teenagers. When KSLQ came on the scene in late 1972, adoption of FM in St. Charles County among my cohort was rapid, even faster than the population growth. Also popular was KADI, doing sort of a watered-down progressive rock format. KSHE was for the "stoners" but, secretly, I listened to it!The biggest incentive for listeners too move to FM as the 70's opened up was the almost uniform limit by the new FM Top 40's to run just 8 minutes of spots an hour. The difference between around 16 minutes on most Top 40 AM stations to just 8 minutes or less on an FM was a huge incentive to move.
The sound quality was just an added benefit.
Meanwhile, KXOK dwindled rapidly. It wasn't just the number of commercials that was grating, the 1960s high-energy jock style was rapidly passing out of fashion and was a turn-off to many of us. For example, I just didn't get what the big deal was about "Chickenman". I thought it was stupid and tedious. That's just one of many things about the Top-40 format.
Edit - I should hasten to add that quality, low-cost made-in-Japan stereo systems were becoming common and, by the time I went to college in 1975, just about every guy had one. Those receivers, often Pioneer (I went with Advent and no AM at all), had AM sections inferior in fidelity while a great deal of attention was paid to FM audio and reception quality. That surely must count as a factor, too.