The real reason - not the Covid thing.
I think it's plain to see how he's been more and more struggling to play, and that's been going on for years. It was visible even during the last tour, or even before that.
If he wanted to reveal any details, he could long have done so, or maybe he will one day. But for me it's perfectly fine if he doesn't, because it's a private and personal thing even for a "celebrity".
He gets my full respect and admiration not only for the music he creates, but also for going on, adapting and finding new ways of being creative and inspiring to a lot of people.