>Charizard: organic popularity
Ever since day 1, Charizard has had an enduring following and legacy despite being on equal footing with the other two starter Pokémon. Even though it was a shitmon in competitive from Gens 1 through 5, people were so insistent on using it that Smogon had commentary on their Charizard sets noting how people were trying to use it in vain. Game Freak finally caved when Gen 6 and megas came about and threw the fans a bone, as a response to Charizard's popularity and cultural impact.

>Lucario: artificial popularity
Lucario on the other hand was shilled hard before it was even a useable Pokémon in a mainline game. It starred in its own movie and was the highest rank possible in PMD Rescue Team, the Lucario Rank. It only got into Smash because of recency bias, since they needed a Pokémon from what was the newest generation at the time. Despite no one actually liking Lucario, continued shilling happens such as it becoming the face of mega evolution in XY. It never had a chance to stand on its own two legs.