The problem is that's not really a comment, it's a string which needs to be evaluated as data, wasting resources while never being used. In any sane language, comments are simply ignored by the parser or compiler.

Because Douglas Crockford didn't want people abusing comments in JSON by turning them into parsing directives, JSON doesn't allow using comments at all, even correctly, so everyone is left abusing JSON itself with workarounds that are objectively worse than just having comments.