why are there more rock layers exposed in the nankoweap canyon than in the north canyon?

Answer 1

Because nankoweap canyon is cut through the very peak of the dome of rock that formed the Grand Canyon and because it is downstream of north canyon therefore the mouth of north canyon has to be above the lowest levels of nankoweap canyon.
The accepted explanation of the Grand Canyon is that the existing Colorado River stayed in place while the rocks in the area slowly rose, allowing the river to carve the canyon over a long time. The further downstream and the closer to the maximum height of the dome, the more layers are exposed.