Is there any reason not to use radiant barrier sheathing in walls of attic room
Yes, if you don't have a way for the heat to escape rapidly. Radiant Barriers aren't insulation. So, if the reflected heat can't get away quickly, like in a wall, the Radiant Barrier won't do anything of benefit in your location since you would want it facing the exterior to repel heat.

It really would only be useful if it were installed as your exterior sheathing, like the roof, to reflect heat back out through your siding or finished surface. Up North it's used on insulation, not sheathing, to reflect escaping heat back into the living space during the heating season.