Answer : Due of inverse relationship between frequency and wavelength.
Explanation :
The relation between frequency and wavelength is:

From this we conclude that, there is an inverse relationship between frequency and wavelength.
That means, as the frequency of a wave increases then the wavelength of a wave decreases and as the frequency of a wave decrease then the wavelength of a wave increases.
Hence, the frequency of a wave increases the wavelength decreases due to an inverse relationship.