I did a little reading and this is NOT the fault of Mackie. It's Apple being heavy handed. On December 19th Apple issued an ultimatum requiring all apps and app updates submitted to the App store after Feb 1st to be IOS7 compliant. OK that by itself doesn't mean they can't not work with IOS 5 or 6, but Apple continued by also requiring all apps be compiled using Xcode 5, which just happens to only spit out IOS7 apps.
I briefly read about some hacks and ways to get the compilers for IOS 5-6 in the Xcode 5 app, but the bottom line is Apple removed native support for it.
My guess is Mackie, and others, raced to get something submitted before the deadline. That would explain Masterfader 2.1 hitting the app store on February 18th, roughly 2 weeks after the deadline, which historically has been the lag time between submission and release off apps to the store.