Earlier quoted context omitted.
The iPhone is not a store. The App Store is a store. Epic wants to be on the iPhone, but not necessarily in the App Store. Ideally, Epic wants to run its own store on the iPhone. In any case, App Store is the only store in town, where the "town" is the 1.5 billion iOS users. If there's only 1 store, with no competing stores, that's an entirely different situation legally.
Think of the iPhone as a mall then - it still gets to pick its tenants by whatever criteria it sees fit (that are not illegal, and fees are not illegal). Epic is not winning this one. Edit: added the bit about fees.
Why?