Because it reflects the red color, and absorbs or hides the other colors in itself.
When anything absorbs all the colors of a ray of light, what color is it?
It is black.
When it reflects all the colors of light, what color is it?
It is white.
Why are the letters on this page black?
Because they absorb all the colors of light, and reflect none.
Why is the paper white?
Because it reflects all the colors of light, and absorbs none.
Why are all things black in the dark?