Until you factor in its typing :P
Steel/Rock does have a few resists, but unfortunately, it has two 4x weaknesses, which severely messes with their walling abilities. That's why Shieldon and its' evo, Probopass, and Aron and its' evos don't do too well in this Fighting and Ground type and move-flooded metagame. Being weak to two of the most common attacking types in the game = OH SHIT.