The missing talus in those canyons is due to advancing glaciers during the ice age. These glaciers pushed the talus ahead of the glacier as it moved through the canyon and eventually left those tumble-rounded boulders hundreds of miles away when the glacier melted at the end of the last ice age.
Yes, you’ll find that talus hundreds of miles away as curiously rounded boulders in the middle of a desert or plain and you’ll wonder how in the world they ever got there.
According to Ancient Alien Theorists it was The Flood that deposited them there.