They did it because a lot of people forget to reg their car since the gov stopped issuing road tax/registration stickers with expiry dates.
For people that borrow a car, or move house, or miss a notice in the mail, none cant tell if the car they are driving is registered or not, they'll be like, "I think it was October, no it was December... no that was the house insurance..."
So in NSW, the state makes $1200 every time an officer stops you to let you know you messed up paying a bill. Far more profitable than printing stickers! And doesn't affect the top 1% as they all have drivers...