Sound synthesis was largely achieved using additive techniques;
Computers were very slow and expensive to use;
Most electronic music was produced using analog technology.
Chowning (and Stanford) tried to interest American companies, including Hammond and Wurlitzer, in the FM technique but they didn't see the potential.
Yamaha eventually signed an exclusive license with Stanford and sold nearly 200,000 DX-7 synthesizers.
The FM patent produced about $22.9 million in royalties for Stanford University (largely via the sale of FM chips for soundcards).
To this day, the Center for Computer Research in Music and Acoustics (CCRMA) is funded from an endowment established from a portion of those royalties.
“The Synthesis of Complex Audio Spectra by Means of Frequency Modulation” [Chowning (1973)]