Here's why:

* Diffraction is the bending of waves around obstacles or through openings. The amount of diffraction depends on the wavelength of the wave and the size of the obstacle.

* Radio waves have very long wavelengths, typically ranging from centimeters to kilometers.

* Hills are large enough obstacles compared to the wavelengths of radio waves.

Therefore, radio waves are able to diffract around hills, allowing them to reach areas that might be blocked by the hill's physical presence. This is why radio signals can be received even when there are hills in the way.

Other forms of electromagnetic radiation like visible light, infrared, and ultraviolet have much shorter wavelengths than radio waves. These wavelengths are too small to be significantly diffracted by hills.