It is because that is the standard place to put your tag when you only have one tag. I have never seen anyone run only a front and I suspect running none on the rear would be a reason for a traffic stop.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.