I think i know the reason.... ENERGY GUIDE LABEL.
the bigger the washer, the better it is for the energy efficiency standards. not exactly for the clothes, but for the waer and electricity use ratings.
Add to that the "average american estereotype consumer" that is a workaholic and HATE doing laundry so they accumulate dirty laundry for as much as they can to do everything at once. Understand it as washing bed sheets, clothes, towels, kitchen towels, whites, coloured and blacks, underwear with tiremarks and even the children, the cat, the dog, and it it fits, even the matress and the husband all together in the same load.
To make things worse, there's the "show off" factor. people that want to show off to their friends only to say "look, my washer is bigger than yours"
And also, the production cost. A 1 cu ft washer costs almost the same as a 5 cu ft washer. What varies according to the size is reasonably cheap (plastic, steel, zinc, glass) and what really costs in a washer is the same (motor with a minimal price change) drain pump, inlet solenoid valves, wiring, control board, and production line staff.