1. It is GPL, so you can't change the license without permission from all contributors
2. It has lots of contributors. And now that I think about it I guess some of those are not even alive anymore to grant any sort of permission
Linus on GPL 3: Linus Torvalds says GPL v3 violates everything that GPLv2 stood for https://youtu.be/PaKIZ7gJlRU