I'd rather turn that question around and ask:
Why do most Sedans not have a rear window wiper/washer?
The only one I am aware of was the Peugeot 406 Sedan of the first 2 years.
It was an option, that was seldomly ordered. My father had one of those
cars, that didn't come with the rear wiper. When ever we had to back up
in the rain I made fun of my dad not having ordered it. What insulted him,
as he clearly would have ordered it, if the salesman had led him to that option...
Windflow on a hatchback (Golf):
Windflow on a Sedan (Phaeton):
So with a Sedan in theory you only have to go fast enough to clear the rear screen from
all debris, but why are most 911s ordered with rear wiper/washer up here in the north?:
I am not young enough to know everything.