Here's why:

* Wave speed (v), wavelength (λ), and frequency (f) are related by the following equation:

v = fλ

* Wave speed is constant for a given medium. This means if the wavelength changes, the frequency must change to compensate and keep the speed constant.

* If the wavelength doubles (λ becomes 2λ), the frequency must halve (f becomes f/2) to maintain the same wave speed.

In simpler terms: Think of a wave like a series of peaks and troughs. If you stretch the distance between those peaks (the wavelength), you'll have fewer peaks passing a certain point per second (lower frequency).