Readability is a browser based reading app that lets you pull an article from a blog or news site and read it without the advertising. Of course, that deprives the source of the content of ad revenue, so Readability turned into a subscription service so they could pay 70% of their income to content creators.
This all happened before Apple got greedy, and as you can see they don't have 30% to give to Apple. It simply won't work.
The Readability app will work just fine with Safari, so as much as they'd like the app they don't actually _need_ it.
via Readability blog