XML was intended as a data exchange format and has comments. Image file formats like JPEG and PNG support comments (and so does SVG by virtue of being XML). Database systems support comments on database objects. It’s really not a convincing argument.

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.

Obviously it's useful in many cases; designing anything like this is an exercise in trade-offs.

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.

Data interchange formats are often used for configuration. It makes sense to have a single source of truth in json if your configuration is consumed by an app.

JSON is a human-readable data exchange format. There is a good reason human-readable data exchange formats are so popular - they can be read and understood by humans! So it seems pretty absurd that a format designed to be read by humans doesn't support comments. If you're really concerned about performance or size, you should use a more efficient binary format for data exchange like protobufs, or heck, BSON (binary json)!