To be honest, I don't think there IS a legitimate reason Game Freak had to making Lucario Steel-type. I have a few ideas as to why, though:
Lucario looks cool already, but pure Fighting-typing is so bland. Game Freak likely put Steel-type on him to make him stand out, and therefore, not be overlooked.
Steel-types are popular among fans, so why not make Lucario this way so that he's appealing?
Fighting is a bad defensive typing. To make sure that Lucario has some defensive prowess, he'd need a good subtype for defending...Why not STEEL?
I think these are all horrible thoughts, and I think it's a dumb thing to do, just because Game Freak wanted Lucario to be popular. Take a look at a few others. Just by their typing, you can tell who Game Freak actually enjoyed designing, and who they put in for fillers.