Adding that capability would raise the complexity of the parser, because comments would have to be made part of the data structure that is built and transformed. For instance, it would be harder to embed the data structure for the JSON in JS objects.
But yes, for a parser that's ingesting data for immediate processing and has no need for comments, there's no discernable win regarding parsing simplicity.
Not to mention N unit tests to cover comment removal verification.
"One interesting story about leaving things out: as we got closer to releasing JSON I decided to take out the ability to do comments. When translating JSON into other languages, often times the commenting piece was the most complicated part. By taking the commenting out we reduced the complexity of the parsers by half—everything else was just too simple."