Because he doesn't like the anti-tivoization clause for his own code.
I suggest watching https://www.youtube.com/watch?v=5PmHRSeA2c8 as it gives a up-to-date view which all the necessary qualifiers in one single talk. I especially like the (paraphrasing quite a bit) "GPLv3 is a fine license, GPLv3 is a horrible license, and I like to overstate things if you haven't noticed. For my own code I just want changes back. GPLv2 is a license that does this and nothing else, which is why I use it for my own projects.".
I suggest watching https://www.youtube.com/watch?v=5PmHRSeA2c8 as it gives a up-to-date view which all the necessary qualifiers in one single talk. I especially like the (paraphrasing quite a bit) "GPLv3 is a fine license, GPLv3 is a horrible license, and I like to overstate things if you haven't noticed. For my own code I just want changes back. GPLv2 is a license that does this and nothing else, which is why I use it for my own projects.".