POKEMON GEN 6 ANNOUNCED

[TomGuycott]

Steel is a pretty powerful type, but in most cases Steel Type takes a lot of work to get them to their strongest point. Lots of them have really high evolution levels (The Gen 5 Pokemon introduced have final evolutions in the 40's and 50's when only having one pre-evolution stage), others require special items in order to evolve (Steelix, Scizor) or require unique conditions (Escavalier, Lucario, Magnezone). In addition, lots of Steel types have quadruple weaknesses, such as Bisharp against Fighting and Ferroseed and every Bug/Steel type against Fire. The exceptionally powerful types, as you said, are Steel/Psychic types, especially when you consider things like Bronzong's abilities giving it immunity to one of its two potential weaknesses.

All in all, though, the Steel Types are a reward. You don't find an awesome Steel type pokemon running around outside the first area of the game; you have to earn them, a lot like Dragon types, though not QUITE as rare as that.