Ghost = new Dragon type much.
Giving Steel types the nerf in Dark & Ghost resistances is Gamefreak's way of attempting to balance out the metagame. Steel types were always superior in walling capabilities (and still are even with the nerf), with Bronzong able to only have one weakness with its ability and Jirachi & Metagross dominating the offensive scene in addition to walling hits without needing any/ much investment in defence. With Gen VI, Steel is now x2 against Fairy, so removing the Ghost & Dark resistance is a way of balancing out a new super effective hit for Steel as well as another resistance in Fairy.
Additionally, with the introduction of new steel types like Aegislash, Mega-Mawile and Mega-Aggron, Steel types have become yet more threatening both offensively and defensively, meaning if the Ghost & Dark resistances were still there, they'd be even more difficult to take down.
This is Gamefreak's way of attempting to deal with power creeping that happens in every generation, which is shown by how Metagross and Jirachi are still coping well in the metagame even with their added weaknesses.