Here's why:

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

v = λf

* If speed and wavelength decrease, frequency must increase to maintain the equality in the equation.

Think of it like this: imagine a rope with waves traveling along it. If you shorten the distance between the waves (wavelength) and make them move slower, you'll have to shake the rope faster (increase the frequency) to keep the same number of waves passing a point in a given time.