Why do hatches have rear wipers & not saloons.

Air flows smoothly over saloon's boot. But for hatches/estates, the swirling wind creates a vortex which accumulates dust on window.

Generally manufacturers don't fit rear wipers on saloon cars because they are deemed to be unnecesary. Most road film that deposits on a car's rear window is caused by the car itself when in motion. In a saloon car (with boot) the rear overhang coupled with a generally steeper rear screen (than a hatchback) minimises the amount of residue on the rear screen.

It is more necessary.
The airflow over the car's body at speed is dependent on the shape of the car. The shape of many hatchbacks and estates (wagons) causes a turbulent zone over the rear glass, so that spray from the road lands on the glass, hindering rear vision. Therefore a rear wiper is useful on wet roads, even if it is not actually raining. This is seldom an issue on a booted saloon.

Orrrr......chey.