Question

Why does the frequency of a wave increases the wavelength decreases

Answer 1

Because their product (frequency times wavelength) is always
the SAME number ... wave speed. 

So if one of them changes, the other one has to change, by the
same factor but in the opposite direction, in order for the product
to stay constant.

Answer 2

Answer : Due of inverse relationship between frequency and wavelength.

Explanation :

The relation between frequency and wavelength is:

$\nu=\frac{c}{\lambda}$

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.