[jbooth]

I don't know a lot, but:

XML looked like HTML at a time when the web was the "next big thing". Like, not a technology on the web, or social media, HTML itself was this big revelation. So a solution that looks like HTML has a leg up.

Then, once there were mature parsers in a lot of languages, server software configured by it, etc, XML had some inertia that takes time to displace.