Because it's not just a snapshot—it has a growing diff to the WHATWG spec, and when they do copy over commits they sometimes do so incompletely. (An example of

Because it's not just a snapshot—it has a growing diff to the WHATWG spec, and when they do copy over commits they sometimes do so incompletely. (An example of this is the HTML 5.1 definition of document.domain: if you try and set it to any host except for the current one it throws SecurityError; this came about as a result of bogusly applying patch from WHATWG spec.) As far as I'm aware, most of the annoyance of the W3C fork is the fact that there's good people spending a lot of time on it (instead of, e.g., working on specifying parts of the Web platform that every modern site relies on that are essentially undefined, like the CSSOM), and the fact that it isn't just a snapshot and that it therefore causes confusion with two different, at times contradictory documents.