Why isn't FM used as an amateur radio emission mode below 28 MHz?

Frequency Modulation (FM) inherently requires a relatively wide bandwidth to operate effectively. A typical FM voice transmission occupies about 10 to 15 kHz of spectrum space, whereas Single Sideband (SSB) requires only about 3 kHz and Continuous Wave (CW) requires less than 1 kHz. Below 28 MHz (the High Frequency or HF bands), the total available spectrum for amateur radio is quite narrow. To ensure that many operators can use these bands simultaneously without causing massive overlap, regulations restrict the maximum bandwidth of emissions. Because an FM signal is so wide, it would easily exceed these strict regulatory bandwidth limits on HF. The issue is not related to transmitter efficiency, harmonic attenuation, or frequency stability, which are all manageable on HF frequencies.