The only concrete reason I could find was this:
In the Generation I games, Psychic-type Pokémon were at the center of a balance issue.1 As a result, the issue was rectified in Generation II, with the addition of Steel and Dark types, an added weakness to Ghost-type moves (as opposed to previously being immune to them), as well as some moves that are strong against Psychic types.
I also searched up a bunch about metal and psychic powers. All I really found was that apparently Silver is supposed to be psychic itself, and that tinfoil is supposedly able to prevent mind reading. Psychic types resist themselves and psychics are generally seen as able to mind read, so even if Game Freak just added that resistance for balance, there are still plausible reasons why Steel would resist Psychic. :P
Also, Steel resisted pretty much everything in Gens 2-5, so they probably removed the Ghost and Dark resistances in Gen 6 so that it would be balanced with the introduction of Fairy types (as Steel has advantage over them). :P
Hope I Helped!