I think back to the video game console days of the mid-90's. Despite not owning any systems until third grade, I remember quite well playing the demo games at Toy's R Us every time a shopping trip allowed for such a detour. When the N64 came out, everyone was blown away by the shift to the 3D graphics. It was a real paradigm shift, and there was a real sense of energy when it the market. Since then, with each new system, there have been improvements, but nothing has really had the same sense of "wow".
The answer, of course, is simple. For the moment at least, all the major innovations have already been added to the phone. Retina display, a good camera, bigger screen, thinner model...these are all luxuries the initial iPhone (intentionally) left out which have been introduced gradually over the past 5 years. At this point, they've all been copied, refined, and made ubiquitous on the market to the point none really impress people. Pay has a chance to make a difference, and Watch may eventually duplicate this, but the iPhone isn't quite as exciting as it was. It's like when you stop getting toys for Christmas and start getting clothes...it makes more sense, but the fun is never the same.
(Disclaimer: To be clear, I am excited about the new phone and I'm sure I'll be very impressed once I finally test it out in comparison to the 5s. If any Android people think this is a rejection of they are sadly mistaken. This is merely a lamenting of the occasional technology-plateau that occurs with products like these.)