Not a lawyer, but I assume because the GPL v3 requires that other people be able to modify the program and redistribute their modified versions. Since Apple are the sole gatekeepers of anyone’s ability to distribute iOS apps to users, complying with the license is problematic.
Long story short, the GPL requires you to grant a license to run the application for any purpose. You can also not add additional constraints to this. The App Store's terms and conditions say that you may only offer the software to be used under the usage terms that Apple gives. These two things are in conflict. Apple generally does not approve apps with a GPL license and most people think it's because of this reason (as far as I know they haven't ever made a comment about it). A few apps have apparently made it through, it seems this has been an error on Apple's part.
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Not a lawyer, but I assume because the GPL v3 requires that other people be able to modify the program and redistribute their modified versions. Since Apple are the sole gatekeepers of anyone’s ability to distribute iOS apps to users, complying with the license is problematic.
Here's the FSF's take on it: https://www.fsf.org/blogs/licensing/more-about-the-app-store...
Long story short, the GPL requires you to grant a license to run the application for any purpose. You can also not add additional constraints to this. The App Store's terms and conditions say that you may only offer the software to be used under the usage terms that Apple gives. These two things are in conflict. Apple generally does not approve apps with a GPL license and most people think it's because of this reason (as far as I know they haven't ever made a comment about it). A few apps have apparently made it through, it seems this has been an error on Apple's part.