Sedans tend to generate a vortex at the separation point on the rear corners of the roof that sweeps water off the window. You can usually see this pattern pretty clearly after driving in the rain- there will be a conical section that stays dirty in the center because it’s outside of the vortices. You get that corner vortex on a car with a more vertical window too, it just doesn’t touch the window. Similarly, on something like a Prius, the rear window is close to horizontal and upstream of the separation point, and doesn’t really get cleared very well (so Prii have a rear wiper).
Vertical rear windows (or any surfaces directly exposed to the wake turbulence) also get a lot more water kicked back up on them from the road.