I don't know why, but that is the way it is. Chickens can lay eggs with a white shell or a blue shell. The blue shells come from the araucana or ameraucana breeds. All other chickens have a white egg shell. Eggs can be coated in some shade of brown or not. A green egg is a blue shell with brown coating, from a chicken that is a mix between a blue egg laying breed and a brown egg laying breed.
Chickens can have skin that is yellow, white or black, too.