Because the Greeks seemed to take more trouble in keeping the shape of the person/object fixed relative to where the stars were, and referred to the same constellations in a systematic manner. The Egyptians played pretty fast and loose with where the stars actually were, and forced the stars to conform to the shape they wanted. The Greeks seemed to regard the stars as fixed, and forced the shape to match it. Also, the Greek 'astrologer' Hipparchos was the first to catalog thousands of naked-eye stars by their positions in the sky accurate to a fraction of the diameter of the Moon.