Apple is not a fan of homescreen widgets. This is much bigger than iOS and predates the iPhone itself. Back with OS X Tiger in 2005, Apple introduced Dashboard as a way to offer the user widgets without having them on screen at all times. It's another example of Apple's preference for clean, uncluttered UI (though some would disagree with that, but thats a debate for another day). Starting with iOS 5 and the notification center, Apple has placed its form of widgets for iOS in an easily accessible, yet ordinarily hidden location. A homescreen clock would go against Apple's current design language with the homescreen.
In summary, Apple will not be allowing homescreen clocks for many years as I see it (though who knows, iOS 11 may change all that).