Don't think he's petulant and childish at all. He probably just woke up in the middle of the night, as he said he did himself, and realised that all the abuse and hassle wasn't worth it.
He's a tough old boot and has held his own through many situations far more serious than this one, but they were probably battles worth fighting to him. I doubt he's cowed or feels bullied, but perhaps he just feels the furore is not worth it? Can't say I blame him if that's the case. He's said himself that it he viewed Strictly as fun and I guess maybe the fun element had disappeared for both John and Kristina with everything being said by certain parties, including, I'd like to add, in forums like these. We don't know that John ever read them but Kristina certainly did. I don't imagine for one moment he quit without her agreement.
Now everyone can move on and whinge and moan about something else.