In Israel there is a 6 day work week and the only day off is Shabbos, so many non-relgious spend shabbos either going to the Mall or going to the beach, the idea of a Sunday is to give these people a day to do these things.
Once you have decided to give a non-shabbos off, you have to realize most business have to more or less follow the world schedual. For example if you are a stock broker or an international trader in import/export you need to work when people are doing the same thing.
International traders work M-Fr not Su-Th or M-Th 10 hours a day.
There is very little movement in US for 4 10 hour days except in jobs like health care, its really hard to work for 10 hours, I know Ive done it many times