XML and its associated formats were just so complex. I remember considering getting a book on XML and it was 4 inches thick. Just for a text-based data storage format...
This is just prohibitively complex. Formats like JSON and YAML thrive because they don't have the complexity of trying to fit every possible scenario ever. The KISS principle still works.
2. These XML books tended to have section on XML and well formedness, namespaces, UTF-8, examples of designing a format - generally a book or address format - all this stuff probably came in to approximately 80-115 pages. Which was what you needed to understand the basis of XML.
3. Then would come the secondary stuff to understand, XPath and XSLT. I would say this would be another 100 - 150 pages, so a query language and a programming DSL to manipulating the data/document format. All this together 265 pages.
4. Then validation and entities in DTDs noting that this was old stuff from SGML days and you didn't need it and there was going to be some other way to validate really soon. Another 60 pages? (and then when that validation language came it sucked, as I noted elsewhere)
5. Then because tech books need to be thick and a 300 page book is not big enough a bunch of stuff that never amounted to anything, like Xlink or some breathless stuff about some XML formats, maybe a talk about SVG and VML, XSL-FO blah blah blah. Another 300 pages of unnecessary stuff.
This is just prohibitively complex. Formats like JSON and YAML thrive because they don't have the complexity of trying to fit every possible scenario ever. The KISS principle still works.