Because usa 20 amp households are limited to about 1920 watts continuous power. I don't think they could practically sell a unit more than 1.9 th/s based on the 1 watt per gigahash.
I wired a 20amp 240v breaker for my ASIC boxes. Switching PSUs can handle 120-240v, so I have all my PSUs and a network switch all running off 240v, which allows me to pull twice the watts on a 20amp breaker.