How would Lucario fight? Would he have any new abilities or moves to offer for the roster? Link, Marth, Pit, and Metaknight all use blades but they have completely different fighting styles.
Popularity is important, but you also have to think about how the character would fight in a battle. What unique abilities can they offer? Ness and Mewtwo are both psychic, but looks how different they are. I mean, Ness has a Baseball bat, while Mewtwo has dark energy.
Lucario is Steel/Fighting, correct? Now having Fox run around, punching and kicking, is fine because he rarely steps outside of his ship into combat. But Lucario comes from a world where characters can use any element and any ability.
Lets compare Lucario with Squirtle, to see who has a more unique style of fighting. Lucario would provide... punches, kicks, maybe some judo moves? Perhaps a blast of energy?
Now look at Squirtle. He has a shell, providing shell attacks. Bubbles, water gun, hydro pump. Not to mention skull bash, and a curly tail. There hasn't been a character who uses water attacks, and his size and water abilities would make him much different than Bowser.
Lucario would be different, but he would be something we have already seen. I know everyone uses physical attacks, but Lucario doesn't have much else going for him since he is a steel/fighting type. Unlike a water, fire, psychic, grass, electric, or even a poison type.
Its just... when I saw the movie, I wasn't impressed with Lucario at all. He just got beat up all the time, so I never really saw anything that would prove he would be a good add in. Mewtwo on other hand nearly destoyed a planet.