The innovation on third party UIs comes in part because developers have near total control over their UI in their apps, free to play around and try new things. Apple, however, doesn't seem to be willing to take any risks at all with the core UI of iOS. Why is that? It could be that the company still believes it nailed it with the iPhone's original UI, but perhaps there's a better explanation.
When Microsoft was at the height of its Windows dominance, it found itself caught in a cycle of sameness with Windows. The need to keep backwards compatibility and the fear that big changes would upset its user base led Microsoft to keep Windows looking the same and behaving the same. It packed on feature after feature, ultimately resulting in a confusing mess of mixed UI metaphors and arcane functionality. The trend culminated in Vista, where Microsoft tried to square the circle of innovating on UI while keeping app functionality the same. It didn't work, and because of the problems in Vista the OS was widely rejected.
Apple has supplanted Microsoft as both the biggest and the most influential company in consumer electronics and technology. Like Microsoft in the 90s and early 2000s, it is taking a very conservative approach to updating its core UI in the name of accessibility and consistency. Apple is keeping the iPhone in a very familiar and safe zone, but does it really need to? Its risky, taking something thats massively successful and trying something new and different with it. Most companies dont do it, but Apple has a reputation built making those kinds of bets. Perhaps it doesnt deserve that reputation anymore.