That's because to do so would require moving the fuel tank filler, which would in turn force BMW to submit the car to new crashtest and certification procedures.
The door, an afterthought in the design process of the Clubman, permits easier access to the rear and is back hinged. It can only be opened when the front door on its side is open.
The Clubman has a longer wheelbase and body than the 3-door hatch Mini, but is claimed to retain the drive feel of the smaller vehicle. BB
