The major factor of different scattering is the ratio of wavelength to the size of particles which are working as microscopic scattering mirrors.
In a sparse particle medium like air, the longer the wavelength, the easier it is to transmit and harder to be scattered. The shorter the wavelength, the easier it is to scatter and more difficult to transmit.
Given the size of mirrors in air, the shorter the wavelength, the stronger the scattered light, the weeker the transmitted light.
In sky, the mirrors are the air molecules. Red light gets more transmission, blue light gets more scattered.
So the sky is blue.
This is my understanding. Anything wrong with my above words, please give advise and corrections.