There seems to be a lot of misunderstanding of apple's position on larger screens. Adding more screen sizes is not as simple for apple and iOS as it is for android.
To understand why, you have to think back to the iPhone 1, and iPhone OS;
I think people today forget how revolutionary that device was; it was a nearly desktop-class OS experience, optimized for touch running in the palm of your hand.
What people also forget, or perhaps never knew, was how little CPU power the iPhone 1 had. And YET it still was so smooth that you would hardly ever notice any stuttering or lag.
Even a few years LATER, android, on MUCH better hardware was nowhere near as smooth.(do people remember project butter?)
So how did iOS achieve this performance on such weak hardware? By making specific software decisions in the name of performance. One of these is the fixed pixel layout of the apps.
The original iOS had no real automatic layout feature (probably due to performance). Nothing was scaled.
Soon the new App Store was filled with apps based on this fixed layout design.
The next step for apple was to increase the screen resolution, to something much sharper. They could have gone with 720p or some other standard, but that would cause problems with the existing apps. They would need to be fractionally scaled, which would produce UGLY results.
So apple went with the logical resolution, 2x in each direction. 4x the number of pixels and a simple 2 times scaling which produce perfect scaling for existing apps.
Then when there was a need for a screen size increase, apple could have added width AND height, but, again this would be a problem for existing apps; so, instead apple only added height so existing apps could be "letter boxed" and not scaled at all.
It was at this time apple added an autolayout feature to iOS and recommended all developers begin using it. However with thousands of apps in the App Store, this transition wasn't going to happen overnight. So apple had to wait until it felt most of the apps had made the transition to autolayout. Then it would be time to allow apple to use any sized screen they wanted.
Which is what we are getting now with the iPhone 6
This frequently happens with software that is ahead of its time; compromises have to be made to bring the future to today, but soon, hardware advances, and those same compromises that made the paradigm possible are now more of a liability, especially if you need to maintain backward comparability.