http://www.macrumors.com/2015/12/01/ske ... xperience/

Their blog post lists a few points, they feel that the Mac App Store doesn’t have the focus and attention that the App store for iOS does. There was also the whole hideousness of the Mac App Store certificate issue, where apps you had bought refused to run (which rang alarm bells about apps “phoning home” to Apple for DRM reasons in some circles).

One big issue that developers have with the Mac App Store is the inability to charge upgrade prices, so they either have to run a discount for a few weeks, provide the new version for free, or create a brand new app and charge for that.

You can read the reasoning for leaving here: http://blog.sketchapp.com/post/13432269 ... -app-store