Well i'm by far not an expert. But a lot of people consider that GPL is an open but maybe too restrictive licence.

Notably, "derived works" using GPL copylefted code must be redistributed under same licence. So basicaly if you consider that Swift is a key component of OSX... This is just not something Apple can accept right now.

Beside I'm not even sure a Licence switch from Apache2 is possible without consent of all contributors. And most definitively a Licence switch can in no way be decided via a Pull Request :P

Feel free to correct me if I missed a point ;)