Because many people work M-F and have the weekend off. Therefore, they must do things like get their car worked on during the weekend, if at all possible. It is a very big inconvenience for most to arrange for rides to/from work when the car is in the shop.
That being said, at my current job I work rotating weekend shifts (I work one weekend, then have two off) and I actually enjoy the times when I work the weekend; I have Thursday and Friday off those weeks, and it's much more enjoyable to go around doing things during the mornings/afternoons when there is nobody around compared to the weekends. If I could have weekdays as my 'weekend' all the time I'd do it, except for the whole 'all of my friends hang out Friday and Saturday night and I have to get up for work the next day' thing.
So I suppose having a rotation is pretty good to keep the balance
