By the way, you actually played AS the pokemon in the game, so that's why they chose Pokemon instead of the trainer for Smash Bros in the first place. Just after years of consideration by the team, Pokemon Trainer became a character... but you STILL play as the pokemon.
Basically, every pokemon has an equal amount of being added, mainly because each pokemon is not truly a main character with no true relevance of the game. You CREATE their relevance to the game. And only popularity will tell whether they'll be added to Smash. (Well, Mewtwo technically has relevance...)
But in Final Fantasy's case, the characters actually HAVE relevance to the story and game. Which is WHY Mog wouldn't be chosen. He NEVER has ANY relevance to the game. (Not counting that mini-fight when they aided you.)
That fact is why, for example, is why a supporting character wouldn't be chosen over a main character.