Because Sunday is a day of rest in most Western countries, as a part of the weekend. For most Christians, Sunday is observed as a day of worship and rest, holding it as the Lord's Day and the day of Christ's resurrection. In some Muslim countries and Israel, Sunday is the first work day of the week.
Because there is world other than office too. So sunday is fixed for your own life and other days for office or whatever.It is mostly off day in majority of countries in the world.