A simple example: if the order of the keys is different, say because of a different json implementation, semantically the document is identical, but you can't apply the patch anymore.
That's why you use a canonicalizing JSON serializer, which you'd also want to use if you were tracking changes to JSON in git, for example.
That's very limiting. So I'm suppose to use the same Json library on various platforms for client as well as server? And what have you really gained?
The OP was talking about the current incumbent, and the solution proposed does not work for web applications, since the JSON.stringify in most browsers do not sort keys. Sure, you could implement your own javascript version, but it will be at least a couple of orders of magnitude slower.
There are plenty of textual tree delta formats too.