Because the first time we saw her using that ability was on animals.
Because Smile users are actually 2 beings in 1 body with a conscience each one, and they can't control that other conscience since it's a different individual, and that individual is a simple animal, but it shares the same body, so controlling a conscience is enough to control all the body.
Because true Zoans don't have 2 beings in 1. And despite having an animal form, in that form they have their normal intelligence (of a human).
And because after so many chapters, there is no proof of her being able to control true Zoans.