If it was always on the same side, filling stations would have to be built so that you could only approach the pump from one direction or the other.

From that point of view, it makes the most sense to place the fill tube in the rear (like in the old days when they put them behind the license plate) so you can approach any tank from either side, but its probably easier to route the tube around the trunk when its placed on one side or the other, especially in smaller cars.

Maybe someone else has another theory.