The UI changes in iOS 7 make supporting both 6 and 7 more of a pain than most previous iOS releases. Certainly possible and not a huge deal, but it doesn't just come down to iOS 7 specific APIs.

Supporting iOS 6 is a non-trivial amount of work for many apps. Furthermore, you also have to test on iOS 6, and on more devices.

All that for a piece of the market that now amounts to 25% and is shrinking every minute.