In rainy season there are many clouds in the sky having tiny water droplets.
When the sunlight is incident on tiny water droplets, due to these tiny water droplets, refraction, dispersion, internal reflection and at the end again refraction of sunlight take place.
Due to which a band is created which contains seven colours in the sky which is known as rainbow.
In other seasons there are no clouds in the sky. Therefore there are no tiny water droplets in the sky. Hence, no rainbow is formed in the
sky in other seasons and so it is not visible in the sky.