Because the Grand Canyon was carved out of a vast reservoir by a river. The river began cutting away the soil, causing a feature we know as a waterfall. this cutting actually began on the far west side of the canyon and worked it's way east. The river that cut out the feature we now know as the grand Canyon flowed out of an incredibly huge lake, nearly the size of Colorado and part of Kansas, (this lake sat over the area where the Grand Canyon now sets, and continued far to the east), where sediment had settled over many millennium. That is where the sediment originated.