Since no lift in the vertical plane is produced by the wings at 90 degrees bank, either the fuselage and vertical stabilizer must hold the airplane up, or it will descend. Aerobatic airplanes with relatively large vertical stabs and rudders and a plethora of power can maintain level 90 degree bank turns for a limited time.
Also, don't forget that "down" is sideways with respect to the airplane, so everything has to work with 0 "vertical" G and 1 lateral G.