If you were "a step from heaven" how would you feel? Hopeful? But what if you started out with the idea that you were already going to be in heaven… And then found out that you weren't, and instead you were a step away? Not feeling quite so hopeful anymore perhaps?
In fact, you'd probably be feeling a little disappointed (okay, maybe a lot—we are talking about heaven after all), and that's pretty much what the title is about: feeling disappointed, specifically about America, which is supposed to be heaven for Young Ju.
But even though the title plays on how much America isn't heaven, it's also about how close America could be to heaven, how hope can exist for Young Ju in this new country… something she definitely experiences toward the end of the novel.