It might be in part to the differing goals of the W3C and WHATWG. To simplify, W3C wanted to stick with versioned specifications like they have always done and

It might be in part to the differing goals of the W3C and WHATWG. To simplify, W3C wanted to stick with versioned specifications like they have always done and to finalise the HTML5 standard at some point and work on HTML6 or whatever came next. WHATWG saw HTML as transformative and wanted to create a standard that both established a future direction and took implementation specifics from vendors into a single "living" specification. DTD's seem to concern XHTML more than HTML, and WHATWG seems to have mostly dismissed the issue of parity between XHTML and HTML. WHATWG wants to treat the DOCTYPE as optional anyway, and is only including it for legacy reasons. This Stackoverflow answer goes into some good detail: http://stackoverflow.com/a/15245834/132164 http://stackoverflow.com/a/15245834/132164