On the higher frequency ranges above 30 MHz or above the upper end of the shortwave band, the signal bandwidth or channel spacing plays a subordinate role. FM signals have a much better audio quality and intelligibility, it is easier to work with squelch, and fading is hardly relevant as a transmission problem.
With a frequency-modulated transmitter, the transmission frequency must fluctuate in a narrow range in time with the modulation; in the receiver, the use of a discriminator (to eliminate amplitude-modulated signal components that cause interference) and a discriminator is necessary for demodulation, and the technical complexity is much higher.
In contrast to VHF broadcasting, which uses a channel spacing of 100 kHz (sometimes 50 kHz), military and PSB communication uses a channel spacing of 50 kHz, which has been reduced to 25 kHz or even 12.5 kHz in newer generations of equipment. However, compromises have to be made with the frequency response or the voice transmission quality with the so-called NBFM (Narrow Band FM).