It is always instructive to begin by looking at the history of highways in the area, in order to understand how and why the current situation came to be.
The early years To this day, vast tracts of land in northern Arizona and southern Utah are miles away from any kind of paved road. But the remoteness of this area was even more pronounced back in the late 1920s, when the US routes were first commissioned. At the time, there was no road worth showing on maps between Green River and Salina UT (along today's I-70 corridor). Nor between Cortez CO and Tuba City AZ (the path of modern US 160). Nor to the south from Monticello UT (along today's US 191):
This image was taken looking south on NM hwy. 469 in San Jon. There, Historic US 66 is now classified as Frontage Road 4118. I was surprised to see that its route marker was a cutout (tap to view a larger version):