Except there were many other reasons, including the fact that FM was in stereo, with a cleaner signal, the music was perceived as better, and there were formats not available on AM. This coincided with the home stereo explosion. And as FM became more successful, the commercial load increased.
I remember in the early days of FM MPX in the Bay Area (when the FM's played the same material as the AM's) several songs sounded so different on the FM vs AM I wasn't really sure they were, in fact, the same song. It was too long ago now to remember the specific songs but there were more than a few of them. It was like the AM version had complete stanza's missing.....or, it was recorded in the wrong key.