Even if we stick to the data exchange format, it would be practical to have comments in examples of data. This would be good for training and learning, for documentation.
This is still possible. There's nothing stopping you from putting comments in the JSON in your file or a webpage. It's just not going to be accepted as valid by JSON parser, which for learning material should be fine.
Obviously it's useful in many cases; designing anything like this is an exercise in trade-offs.