An interesting perspective on why JSON has become so popular compared to XML from MIX Online: The Rise Of JSON:
There is the simple fact that JSON is smaller as a payload than XML. And no doubt JSON is less verbose than XML. But there’s much more to it than just size. The crux has to do with programming. JSON is natively tied to JavaScript. As an object representation of data, it is so easy to work with inside JavaScript. Its untyped nature flows perfectly with how JavaScript itself works. Compare this to working with XML in JavaScript: ugh. There’s pretty fascinating piece by James Clark called “XML vs. The Web” that really dives into this.
JSON’s untyped nature flows with how the web itself works. The web does not seem like typing; it doesn’t like schemas; it doesn’t like things to be rigid or too structured. Just look at the failure of XHTML. A beautiful idea for the purists, but for the web, its lack of adoption underscores its platonic ideals.
I’m odd I guess since I still enjoy working with XML. Considering our API integrations at StatsMix though, JSON is definitely the prevailing format.