Because the objects in the foreground (the earth, White) are orders of magnitude brighter than the stars. If the camera were set up to photograph the starfield, White and the earth would be overexposed. Conversely, if the camera is set up to capture the foreground objects, the starfield is too dim.
Thanks for the info. It would be interesting to know what this exact picture would have shown (starwise) if it had been taken on the dark side of the earth.