Just parsing XML alone is hugely painful, let alone implementing the rest of the stuff like XSLT, namespaces, validation, xpath, etc etc. Plus, once you've done this, you still need a natural way to map this into domain types, or you need to force people into a visitor pattern or some other awkward deserialization technique. JSON just requires a single JSONValue sum type.
XML has its place, but it'd have to be a pretty extreme case of needing rigor or a tree where I need to be able to peg arbitrary attributes onto the nodes in order to see it as attractive. Most APIs won't benefit from all of XML's features.
For instance I maintain a podcasting/rss feed library and XML (And more importantly, the way people publish invalid xml) makes me really wish they had gone with a different format in the day that was harder to fuck up.