“I’m cool and I don’t care what you think” says Mega Lucario.
– Pokemon X/Y Pokedex
Mega Lucario gets the ability Adaptability, which powers up STAB moves from 1.5x to 2x, so moves like Close Combat and Aura Sphere would be boosted.
Steel being super effective against Fairies means that it would be actually useful for Lucario to have a Steel move. Steel resists Fairy, but Fighting is weak to it, meaning Fairy moves would hit Lucario neutrally.