Why XML?

So, I've been playing around with Ajax and rich client web stuff lately. I wonder why not just use Javascript to pass data instead of XML? I know there's frameworks to read/write serialized XML objects out, but why not Javascript? I found that Javascript was easier to traverse the objects and I was able to add behavior (of course, you need to be careful on pushes to the server to insure against malicious code, but I digress). So, I get the power of objects and not just data structures. If you're pulling information, Javascript seems like such a better match. It's a more natural fit for the client side and it's not hard to generate/parse javascript code (if it's just data as in XML). XML traversal is kludgy at best on the client side. Maybe an XPath framework would be nice, but why go through the trouble? You have a perfect representation that fits naturally with no fuss. I wonder if its another case of we-have-to-use-XML-because-it-is-portable.