JSON is one of the most dominant tree-data formats on the web because of the awesome balance it has between complexity and simplicity. JSON fits the bill nicely.
Actually... it's one of the most dominant tree-data formats on the web because you don't have to bundle a parser with your client and then figure out the idiosyncrasies between your serialisation model and your language's object model, the browser gives you one for free and JSON is your language's object model. It's a lot more fiddly to deal with in languages which don't have a K/V map as the primary object model.
