I know it's the official reason, but it's also a really bad one - nothing prevents current JSON parsers to add some weird syntactic rules inside plain strings (similar to "use strict" in JS), but you don't see that happen. It's always been a completely hypothetical issue.

The only real reason why comments would be problematic is that they are a pain to preserve in a consistent way when editing a file, and thus would require extra work in parsers / serializers. Still, it would be worth the cost imo.