It failed because nobody wanted to use it. Nobody built a compelling product from it, nobody wanted to author the data, nobody cared to use it when anyone did. It wasn't killed, it never got started because it never had a useful purpose.

I remember learning XML and XSLT from library books in the late 90’s and found it interesting, but couldn’t find a use for it - I wasn’t really handling data in any meaningful way back then, it was all just hobby learning, but in the decades since I’ve touched XML _at all_ maybe three times? Once was a Salesforce integration I wish I could fully forget, the other times were client one-offs when they had a few things stored in XML that we needed to pipe into WordPress.