Because the eye is most sensitive to green. So in order to save processing video is encoded as a "base" green as luminance. The eye is most sensitive to luminance (how dark/light something is). The other colors, blue and red are then encoded using less data per color. This creates what is called YUV or YCrCb encoding. In short, it is a form of data compression capitalizing on our green sensitivity. see https://en.wikipedia.org/wiki/YUV