Here's why. Try to sell an app today that will run on an iPad 1 or a touch 2nd gen. You can't. Apple requires that ppl buy new hardware.
Note how most current apps in the store still say "Requires iOS 4.3 or later". Even if they required iOS 5, which they probably will after iOS 7 is released, apps could *still* work on an iPad 1 if the hardware can handle it.
Apple must be concerned about their loss of market share to even bring this up.
As a part time developer, I'm interested in knowing the iOS breakdown of active users. I actually had no idea it was this high; for this reason I might not even bother having my apps support even iOS 5 moving forward as it wouldn't affect many people.
Hardware "fragmentation" is only an issues in the context of software developers. Users mostly benefit from it because they have a huge choice of hardware.
Hardware fragmentation can limit user choices in ways as well. Huge variations in hardware also equals smaller numbers within specific models, which equals lesser choice of 3rd party accessories, at least when the specific physical design is involved. And then you get stuff like games optimized only for a specific GPU.