calibre cannot be in the Mac Store due to not meeting a number of their requirements. To name a few that might be issues:
- Apps that are "beta".
- Apps must be packaged and submitted using Apple's packaging technologies included in Xcode - no third party installers allowed.
- Apps must comply with all terms and conditions explained in the Apple Macintosh Human Interface Guidelines.
- Apps that do not use system provided items, such as buttons and icons, correctly and as described in the Apple Macintosh Human Interface Guidelines will be rejected.
Not to mention that Apple's app store does not allow for compliance with the GPL (the license calibre is distributed under). GPL apps (VLC) have been removed from the iOS store recently due to this.