Because on Sunday there is no big rush to be somewhere. There's no rush hour traffic, and after church, some people just kind of meander down the road. It's not something I have seen much of in the past 20 years. These days people do everything with an intensity that is really not necessary.
Because people get relaxed and peaceful on Sunday, when they get away from their daily routine, they don't hesitate to go hence drive slowly. Beginner at driving is like drivers on Sunday in a sense that they drive slow and keep traffic signals.