Because the Earth is probably heading towards the other side of the sun -- i.e. the sun is not behind the earth (edit).
Another edit: here are the positions of the planets on 19 Jul 13
If the probe is right behind Saturn, then the Earth and Venus should have almost their full disc illuminated somewhat obliquely and Mars is more or less fully facing the probe from the back side of the sun.
Here is a close up of the inner planets: (the probe would be in the bottom left corner)
Planet position images from:
Solar System Live