Subaru has been getting its butt kicked in WRC rallying of late, losing out most often to hatchbacks from Ford and Citröen. Petter Solberg, Subaru's WRC rally ace, convinced the brass at Fuji Heavy Industries that a five-door hatch was the way to go this time, citing aerodynamic benefits and "better handling."
In fact, despite sitting on the same 103.3-inch wheelbase as the new WRX sedan, the five-door's overall length is 6.5 inches less, representing a substantial reduction in polar moment of inertia, which is a big chunk of the "better handling" part of Solberg's argument.
Since the STI exists to homologate the car used in FIA WRC and Group N rally competition next season, that's why we've got an STI wagon, er, hatchback today.
Besides, in STI garb, the five-door Impreza body actually looks mean. Substantially wider fenders are rendered in steel on a unique unibody and complement a track that's wider than that of the WRX by 1.3 inches in front and an amazing 2.5 inches in the rear. Last year's STI was some 1.5 inches slimmer.