Ask any canyon photographer when the Grand Canyon looks its best and they'll give you the same answer: the last hour of the day, when the light goes sideways, the shadows carve the side canyons into relief, and the rock cycles through orange, rose and purple. Most day tours are heading home by then. This one is built backwards from sunset — the day's viewpoints are sequenced so you're standing at the right overlook when the show starts.
You'll still see the full South Rim circuit; you're just seeing its finale the way it deserves to be seen.