Even when you compress XML, it's still around 10% bigger than JSON, and around 40% bigger than protobuf (not endorsing either, just examples). Furthermore, making the compressor work harder isn't free. The time to compress and decompress XML is roughly 2x higher than with JSON, and if you decompress the data before using it, XML will still hit you wrt RAM usage.

All of these would be fine if XML offered you some really good advantage over alternatives, but as far as I can see, it doesn't. It just eats up CPU/memory/bandwith/keystrokes for no reason.

XML does offer some really good advantages when it comes to storing strings and sub-documents. I would absolutely not want to write any kind of document in JSON, but XML has been used successfully for a great many document formats.