Ian Hickson, the editor of the WHATWG spec at the time, acted as editor of the HTML5 standard at the W3C for a while after the two groups agreed to work together. However, the committee had chairs who could override the editor.

However, despite a couple of years of effort working together, the W3C process allowed for a lot of people to raise objections that re-litigated a lot of things that had already been decided in the WHATWG process, or just didn't have implementer support, or whatnot. This led to the HTML5 draft specification being stale, as these objections held up migrating the editor's draft (which was the WHATWG specification) to the TR on the W3C site.

So lots of people who still saw the W3C as the "official" source of HTML were brought to an out-dated copy of the standard, because publishing more interim drafts was held up with all kinds of bureaucracy; and the W3C objected to linking to the WHATWG copy to suggest a more up to date version with bug fixes, so there was a fight over this.

The combination of the W3C's heaviweight process making it easy for lots of people to raise objections to slow down the process, and having the ability for those objections to be escalated above the editor, eventually made Hixie give up on editing HTML5 and just go back to editing the living specification.

The thing is, a specification only makes sense if it's actually implemented. Lots of non-implementers raising blocking issues on wishlist features, and then having to take the time to formally resolve all of those issues, does not make for a productive environment; and when the resolutions of those issues are escalated to chairs of the group or higher up in the W3C against the support of the implementers, it really hampers the process of coming up with a productive spec.

By the way, I haven't followed this drama in a few years, but taking a look at what's happening now, it looks like the W3C is essentially just plagiarizing the work of the WHATWG.

Features are generally discussed in the WHATWG, or implemented by browsers and then proposed, and the spec writing goes on there. After the spec is reasonably well worked out, the W3C is copying and editing some of the text into their standard.

Now, the WHATWG spec is under a Creative Commons attribution license, and the W3C does provide a small attribution in the acknowledgements section, so they are not violating copyright.

However, what they are doing essentially amounts to plagiarism as they are presenting themselves as the source for the standard. The introduction to the standard doesn't indicate that the actual work is going on in another group; they invite people to make comments on the W3C's GitHub. This is confusing, it gives people an out-of-date view of the standard, and it seems to be a move to make the W3C seem to still be the relevant authority when it's basically just cloning the standard from the WHATWG, but with enough wording and formatting differences that it could conflict and is hard to tell when it would.

Alternatively, they could fork the standard but do so more in the way that distributions package packages; take what's from upstream, have a separate set of patches that they apply on top that make it clear what the differences are. For instance, those patches might apply their layout, their disclaimers and the like, possibly disable some things that they think are underdeveloped or contentions and likely to change, and otherwise mostly just freeze the text. They could push any patches that they thought were for meaningful differences that they've fixed to the upstream project. They could properly attribute the WHATWG spec as the original source at the very top of the article, and list the editors of the WHATWG spec as the primary editors and the people doing the W3C release as maintainers of that particular fork.

But instead, they are listing as editors people who are basically just doing light paraphrases of the WHATWG spec.