Lessons: Use Cases matter, and programmers (the users in this case) will choose tools that are both simple, in that they are not complicated/over-engineered, and easy to use, requiring little setup and code to accomplish a task. For parsing data with PHP, constrast using something like SimpleXML or DOMDocument (which is light-years better than where we were in parsing XML just 5 years ago), to just doing json_encode() or json_decode().
In particular, JSON shines as a programming language-independent representation of typical programming language data structures. This is an incredibly important use case and it would be hard to overstate how appallingly bad XML is for this.