Because thats when most of the (professional American) football games are on?
But seriously, it makes sense in a secular way. Most people in non-customer service type jobs work Monday through Friday, normally. Those days are out, leaving us with Saturday and Sunday. Saturday is acceptable, but I think a stronger argument could be made for Sunday. Since most people would start their work week on Monday, at an absolute bare minimum, they should have the one day off before to rest up before starting the new week.
That said, I think there is a strong case for seriously disincentivizing employers requiring their employees to work 7 days a week, but that might not be practical for businesses that are open 7 days a week. In those cases, I think the day of week could be arbitrary chosen for/by each employee.