Why is it easier to read black text on a white background than to read green text or text of any light color?

Because the light colors will only work on a dark background, as dark colors will only work on a light background. If light color were on the white background not only will it be nearly impossible to read, the letters with have to be much larger for older people to read or see. Therefore, Black letters on a white background is most common. But a Black background with white letters could also work, kind of like a chalkboard.