I started this post wanting to get an idea of what the layer was above the Kaibab to better understand why the Kaibab was so neatly scoured and left bare for such a great distance. The layer above was the Moenkapi layer, known as the Chocolate Cliffs in the Grand Staircase, which is made up of siltstones and sandstones, and siltstones at least are softer than the limestone of the Kaibab, which seems to explain it.
Another couple of good diagrams of the area. The first one shows all the layers from the bottom of the Grand Canyon to the top of the Grand Staircase, spanning the entire "Geologic Time Table," the second shows the Grand Staircase only. You have to click twice to enlarge the first one enough to appreciate it:
The above diagrams are from this geology blog.
These show the Chocolate Cliffs as the next layer up from the Kaibab -- made up of siltstones and sandstones according to the Utah Geological Survey website:
Kaibab Plateau: the southern and lowermost tread forms the north rim of the Grand Canyon in Arizona and extends northward into Utah. Kaibab Formation limestones.So the limestone Kaibab is harder and the softer siltstone-sandstone Moenkopi / Chocolate Cliffs formation just above it got scoured off completely wherever the Kaibab is exposed, which is all over the area from the Grand Canyon up to the Grand Staircase.
Chocolate Cliffs: the first major riser is made of chocolate brown-colored siltstones and sandstones of the Moenkopi Formation. Forms a slope below the more resistant cap of light gray conglomerates and sandstones of the Shinarump Member of the Chinle Formation.
...The alternating configuration of cliffs, terraces, and slopes is due to the varied erosion rates of different rock types. Harder rocks, such as sandstone and limestone, erode slowly and make up the cliffs and terraces. Softer rocks, such as shale and siltstone, erode faster and make up the slopes.