There are two answers to this question.
First, the App Store for iPhone/iPad and the Mac App Store are two different platforms independent of each other. It is not possible to distribute a single app for iOS (iPhone/iPad) and for Mac OS.
There is a good reason for this separation between iOS and Mac OS App Stores. The two technologies are quite different. Even though Mac OS and iOS share the same programming language (Objective-C), developing an app for iOS and an app for Mac OS are two very different tasks. This is because of the distinct graphical user interfaces, amongst other things.
Because of these differences, and because we pay a great deal of attention to the finer details of each app, the amount of work to build oneSafe for Mac was very significant. We actually spent one year developing the app for Mac!
Secondly, we want our company to thrive and to be around for a long time into the future, to continue delivering great apps and other products!
So, as a result of all this, we’ve chosen to put oneSafe for Mac on sale at a price that we think is competitive and reasonable to compensate the significant R&D effort that we made.