In a post to the WHATWG list, the editor of the WHATWG specifications explains:
More recently, the goals of the W3C and the WHATWG on the HTML front have diverged a bit as well. The WHATWG effort is focused on developing the canonical description of HTML and related technologies, meaning fixing bugs as we find them adding new features as they become necessary and viable, and generally tracking implementations. The W3C effort, meanwhile, is now focused on creating a snapshot developed according to the venerable W3C process. This led to the chairs of the W3C HTML working group and myself deciding to split the work into two, with a different person responsible for editing the W3C HTML5, canvas, and microdata specifications than is editing the WHATWG specification.