Why do you think Steel has so many resistances?
My theory is that they wanted to balance out other types by giving them a type that's resistant to their moves.
For example types like Dragon had so few types that resisted Dragon moves so Steel is made to resist it? And Steel is a strong metal so they may have thought it suited having lots of resistances but its unbalanced how many Resistances it has compared to other types.
I plan on writing about this in my Pokemon Strategy blog when I see what some people think, my Pokemon blog can be viewed by clicking on this link: http://pokemonstrategyguy.t15.org/