I would naively claim that unions have power because they are physically localized, and represent similar people (income, standard of living, laws, etc), and create a very physically localized skill vacuum when they strike, that applies immediate and significant pressure.

App Store developers are sprinkled all over the world, so not too much is consistent between them. Getting enough of these varied people to collectively pause their income, and allow their competitors to flourish, long enough for Apple to even notice/care, is probably much harder. I think it would be even harder to present the applications failure in a way that an average consumer would sympathize with or even understand.