Comment by WorldMaker

They weren't sidelined because they had bad ideas (XHTML 2.0 had a lot of great ideas, many of which HTML5 eventually "borrowed"), they were sidelined because they still saw the web as primarily a document platform and Google especially was trying to push it as a larger application platform. It wasn't a battle between the ivory tower and practical concerns, it was a proxy battle in the general war between the web as a place optimized to link between meaningful, accessibility-first documents and the web as a place to host generalized applications with accessibility often an afterthought. (ARIA is great, but ARIA can only do so much, not as much of it by default/a pit of success as XHTML 2.0 once hoped to be.)