Why Is The Sky Blue

Because that's the wavelength of light that is scattered in all directions when it hits the molecules in our atmosphere.

If a different wavelength were so dominantly scattered, that means our atmosphere would be made up of different molecules, which probably means everything on Earth that lives off air would be living off something else, which means everything would be completely different.

A green sky could mean the atmosphere is made of a very thick gas, so we're all 11' tall and offensively hirsute with incredibly deep voices.  Trees are made of iron.  Hard ocean waves crash on the shore like pieces of slate.

Yellow sky might mean a very thin gas atmosphere and scorched planet where nothing grows more than 8" high because the average temperature is 120ºF.  

We are what we breathe.  The oxygen we all need probably informs our physiology.  If we were made to breathe something else, we'd likely be very different beasts. 

That's why the sky is blue.  Now go to bed.