Hatchbacks often have more vertical rear windows, whereas normal sedans have a more slanted rear windows. The more vertical windows catch dirt and dust more easily than a sedan.
Because hatchbacks have more fun.
There was a time... I leased a Ford Fiesta.. And this was why.
My best guess:
Give the greater slant of the window, water runs off too slowly to provide adequate vision in the event of a heavy rain. It's a safety issue.