I don't think the majority of the feature exclusivity is hardware, it's about differentiating the devices. iOS hardware seems to have hit a convergence point. Although the specs are different, the perceived difference due to the hardware is slight (if there even is one). That means Apple needs to do something else to encourage people to buy a more expensive device, software is now the discriminator instead of hardware.

The iPad 1 corroborates this. It's got the CPU power of the iPhone 4 and the RAM of the 3GS, both of these devices get iOS 6, the iPad 1 does not. There's no apparent technical limitation, they're doing it to differentiate product lines.