Because the suns light reflecting off of the oceans and seas I think. I might be wrong or it could be the Rayleigh effect scattering light and considering the color blue is more dominate making the sky blue.
The light from the sun reflects off of water particles in the air creating color. The other colors move slowly so they get sucked up into the air, but blue moves fast so it doesn't. and therefore, the sky is blue
A combination of two competing effects: 1) Rayleigh scattering of sun light off small atmospheric particles. The scattering cross section is quartic in the light frequency, so that violet light reflects nearly 16 times as much as red light. A flat power spectrum would make the sky violet.
2) the sun's light peaks in intensity in the green part of the spectrum. Without Rayleigh scattering the sky would be green.
The two competing effects are 1) pushing towards violet, and 2) pushing toward green. The combination of the two settle at blue, in between violet and green on the spectrum.