Honestly, FF is a mass market game, and the biggest international brand that Square-enix has.
The more linear, story driven game you have, the larger the potential audience.
Because the story is not too complex, the game is easy point A to B with a map pointing the way, and the fighting mechanics, while they have plenty of depth, are somewhat streamlined, non-hardcore people are more likely to play and beat the game.
I would expect more experimental, less controlled gameplay in NON-FF titles, actually.
FF12 basically proved that most of the audience can't handle too much freedom.