The world book has been in development for a long time. It was first announced in 2006 for publication in late 2007/early 2008, back when Martin was deluding himself that he could finish Dance
very rapidly after Feast
. Then it kept getting pushed back along with Dance;
by June 2011, when Martin had finished Dance
and was turning his attention to various other projects, the world book was "almost three years late" and "need[ed] to be finished and delivered by year's end." That was, I would guess, the big reason why he reported around that time that he wouldn't even be starting The Winds of Winter
until early 2012; he was hoping to finish the world book instead. Obviously that didn't happen; as recently as mid-September of this year he was still working on it, reporting (oh, the surprise) that the new material was taking, and becoming, longer than he had expected.
Looking now at the subforum
of Westeros.org where the world book is being discussed (including by the co-authors and forum owners), I see that the text of the world book had been finalized by the end of last year and they were just waiting on artwork... but then while the artwork was being completed Martin decided to add more material, including the stuff he's been working on recently. The map book, a fairly recent development, ended up taking the publication slot originally planned for the world book, which is now on track for release sometime late next year.