Browsers had XML parsers before they could handle JSON directly, and at the beginning there were complaints that JSON was harder to use for that reason. The reason why JSON won rapidly even for backend apps which never loaded it in JSON was ergonomics: every part of the XML world from the parsers to XPath/XSLT/XQuery to the rat’s nest of standards was plagued by the hairy-shirt “this is hard and should feel hard” attitude that has thankfully become less common. I saw so many people just burn out in the entire ecosystem because they got tired of unhelpful errors, pointless usability bugs around namespaces, low-quality or missing examples, and especially how common tools just stopped getting improved.

I maintain that the format would have been far more popular if all of the effort spent on standards work after the turn of the century had been suspended and the time directed to fixing things like the usability of namespaces in almost every parser, and hiring at least one person to work on libxml2 so developers could actually use features which shipped after 1999. Unfortunately it seemed like there were a ton of architects who really wanted to spend time building castles in the air and they just seemed to assume that someone else would do the boring parts of implementing it, but those people all jumped on JSON pretty quickly. I worked with a bunch of people who weren’t developers and the cycle of initial enthusiasm fading into “doesn’t this kind of suck?” with XML was depressing to watch having seen so much initial promise.