Why would I need the dump in JSON? XML is for sure uglier but at least there are efficient SAX parsers.
More seriously, it has been gaining popularity because of it's direct relationship to built-in structures in most programming languages.
For example: XML doesn't have a concept of arrays, but accomplishes storing collections in it's own special way. It's up to the parser to determine how this should be referenced and interacted-with, while with JSON there's no question: it's an array.
import json
js = json.loads(wikidump)
XML? No clue how to manipulate it properly. I could figure it out of course, but now I don't have to. JSON + Python just works™.