Reason Why FM radio stations have a limited broadcast range
FM (frequency-modulation) radio has several advantages over AM (amplitude-modulation) radio. The first is quality of sound, which is much clearer due to the lack of static and distortion. Also, it is possible to broadcast in stereo over FM bands, which is of particular importance when broadcasting music. The drawback to FM stations, however, is that they have a limited broadcast range.
FM radio waves travel at a higher frequency (and smaller wavelength) than do AM radio waves. Whereas AM radio waves are within the frequency range that is reflected by the Earth's upper atmospheric layer called the ionosphere, FM radio waves are not. Instead, FM waves slip through the ionosphere and become lost in space.
What this means is that FM radio cannot bend around the curvature of the Earth.