While I shouldn't speak for JSON Feed's developers (Manton Reece of Micro.blog and Brent Simmons, original developer of RSS/Atom reader NetNewsWire), their argument basically boils down to "developers seem to be better at constructing valid JSON than they are at constructing valid XML."
I know a lot of HN folks will say but you can just construct perfectly valid XML with library such-and-such or API this-and-that and, yes, I'm sure they're all absolutely, positively 100% correct. But in my real-world experience, Reece and Simmons seem to be correct in practice. Given that Simmons wrote NNW, worked at the now-defunct server-side RSS company NewsGator, and is now writing a new RSS (and JSON Feed) reader, I'm inclined to believe that when he says JSON Feed "reflects the lessons learned from our years of work reading and publishing feeds," he means it.
I know a lot of HN folks will say but you can just construct perfectly valid XML with library such-and-such or API this-and-that and, yes, I'm sure they're all absolutely, positively 100% correct. But in my real-world experience, Reece and Simmons seem to be correct in practice. Given that Simmons wrote NNW, worked at the now-defunct server-side RSS company NewsGator, and is now writing a new RSS (and JSON Feed) reader, I'm inclined to believe that when he says JSON Feed "reflects the lessons learned from our years of work reading and publishing feeds," he means it.