Why is lucario a big deal to everyone? he evolves, so chances are he's not a ledgendary. Heck, he even got his own movie!(along with mew)
Nobody can answer to these questions. It's a little bit like Pikachu's topic. Nobody knows why Pikachu is so famous since he's not a legendary, he evolves and he's not one of the stronger Pokemon.
But Lucario is really strong! Have you seen it's base stats? It is an awesome sweepin' beast. But not a legendary.
Ιf a pokemon isn't strong enough, that doesn't mean, that isn't suitable for protagonisting in a Pokemon Movie....... I hope you agree with me......