Deserializing somebody else's XML to some usable internal data structures generally requires writing serialization/deserialization by hand and it is always a pain in the ass. On the other hand, JSON basic structures map to reasonable internal representations, so I often can simply iterate through the structures coming as-is from the parser library.

I mean, if the same webservice is offering the same data in both XML and JSON format, chances are I'd have to write less code for handling the JSON endpoint. For a client written in e.g. Java both cases may be pretty much equal, but for dynamic languages like Javascript or Python, the difference is significant.