We’d all been burned on enterprisey balls of mud like XMLRPC and then SOAP. When JSON-over-HTTP came along, it was a breath of fresh air. Look, it’s readable! It’s easy to parse! It’s trivial to generate! It only had like 5% of XML’s features, but that was the point. Mere humans could read it and there was no temptation to stick something in an attribute instead of a value.

Seriously, it was revelatory.

Edit: Also, lots of common computer interactions — I’d go so far as to say most of them — are only passing around a few values in a simple data structure. JSON’s good for that. I wouldn’t want to necessarily use JSON to represent an office document, but that’s not what I want to use it for. “Hey API server, give me information about song 1234.” “OK, that’s ’Hotel California’ by The Eagles.” You can definitely describe both question and answer with XML, complete with schemas and all, but does that get you anything over {“name”:”Hotel California”,…}? Not really.

Consider how much you can accomplish with POSIX, and it doesn’t even have named arguments. There are times when XML’s complexity is a benefit (I guess; I’ve never seen one but I don’t doubt they exist). Most of the time, it’s just overhead for the sake of overhead.