Here’s what Sakurai said about the return of the character:
The heroic Aura Pokémon, Lucario, joins the battle!! We’ve increased the influence of Aura this time, so a damaged Lucario is truly a force to be reckoned with.
Lucario joined the battle for the first time in Super Smash Bros. Brawl, which was released six years ago. The production staff of Brawl worked really hard to expand the character roster–without their efforts, Lucario probably never would have joined the battle.