It's pretty much an effort vs. payoff thing; the dev environment get nicer each iteration, and iOS < 6 is in a fractional minority.
According to a company called Mixpanel Trends [1], iOS 7.x is currently on 89% of all Apple devices. iOS 6 is just under 10% and iOS 5 and before is under 1.5%.
Since you can't downgrade and devices ship with the latest version, that means that anyone running iOS 5 or iOS 6 is also running an older device with lower performance. If you're doing a resource heavy game it may not even run well on those devices even if you did work to support them.
Even finding a way to test software on iOS 5 or 6 is a huge pain. Apple makes it very difficult to test on anything other than the latest version unless you happen to have a device with that OS laying around.
Yesterday, I tried to pay and download Threes, but it said it requires iOS 6. I never upgrade my iOS devices after too many screwups from Apple. So not sure if I will ever play Threes.
I don't know much about iOS development, but I wonder why a game which is basically a 4x4 grid doesn't work on every single version of iOS. 1024 works just fine.