19th century novelist Horatio Alger has become synonymous with the great American "rags to riches" story, in which the impoverished hero, through hard work and dedication, eventually makes something of himself. But when readers actually tackle one of Alger's novels (all of which have essentially the same plot), they're often surprised to find that the protagonists don't pull themselves up by their bootstraps, but are rather rescued from penury by an wealthy older gentleman who by chance notices the hero in the midst of performing some good deed.
What's this have to do with Apple? Stick with me for a moment. Apple's iTunes Store and App Store are sort of exemplars of what's become known as the Long Tail: marketplaces with thousands or millions of products that, being digital, cost effectively nothing to store or reproduce and can be sold forever; thus, the owners of the individual apps or media can make small but steady amounts of money indefinitely, while the retailer (Apple) takes its cut of every sale and gets rich.
And yet this model is upended when one song or app in the store becomes unusually prominent. Apple's commercials, for instance, generally include impeccably selected songs that are tailored for indie hipsters' tastes but are also more than palatable to the mainstream, and their exposure made big (or bigger) stars of Feist and Yael Naim. So it goes too for App Store programs: the creators of Classics, an e-book reader that features a number of public domain novels, shot up an incredible sixfold when it was briefly featured in one of Apple iPhone TV spots.
As is the case with the lucky young men in the Alger stories, it's not that these songs and applications aren't worthy; it's just that their selection depends on circumstances beyond the developer's or artist's control (namely, that you catch the eye or ear of Apple's advertising folks). It's a great win for those who can get it, but it's not really a business model you can plan for.