Wild guess: it's because he's 65. Many of these severe genetic diseases kill their victims at relatively young ages. Ethical researchers are thus far being sup

Wild guess: it's because he's 65. Many of these severe genetic diseases kill their victims at relatively young ages. Ethical researchers are thus far being super-cautious about modifying the genome of someone who's likely to have children. While the article doesn't say, I wouldn't be surprised to learn that he has explicitly agreed not to have any (more?) children.