For one thing, Trolls: World Tour was a sequel that many people (notably kids) were waiting for for a long time, while Mulan is a live-action remake. Big difference! Also, Mulan is PG-13, which already limits its audience to a certain extent. And then, you have the fact that a lot of the live-action Disney remakes haven’t really been all that great, so there really isn’t much of an incentive to have to see this one, unfortunately. If anything, Black Widow is more like a sequel (or, like a prequel, rather), since all of the MCU movies are interconnected. So that's another reason why it would be a better pick for this new experiment than Mulan.