Let's start by discussing why XML is necessary in the first place. It's easy to make the argument that HTML enabled the explosion of the Web. Reading HTML in a Web browser is so simple and intuitive that just about everyone grasps it instantly. Logical layout makes HTML documents portable. This provides a great deal of device independence.
Hypertext forms webs of knowledge.
HTML forms a framework for composite documents. They wanted structured text with some simple outline capabilities, simple hyperlinks, primitive font control, and maybe some pretty pictures and colors, and that's what they created. It was simple, elegant, and useful. The HTML specification has ballooned to enormous size with the addition of such features as scripting, frames, layers, tables, forms, style sheets, objects, applets, and on and on. HTML is set in stone.