A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light. When we look towards the sun at sunset, we see red and orange colours because the blue light has been scattered out and away from the line of sight.
Because spread

Due to scattering of light
Because in the surface of Earth seen blue colour from atmosphere then blue colour reflected in sky. So the scattered blue light enters our eyes.
When the sunligth passes through the atmosphere the fine particles in air scatter the blue colour (shorter wavelengths) more stongly than red . The scattered blue light enters our eyes .
Becoz of scattering of blue and violet colour the most