Well, George has admitted that it all got away and he couldn't stop world-building or staying within the limit. But when you write Epic Fantasy, everything tends to be Epic, even the length of the books. This same thing happened with Rowlings, as her books grew towards 100k and then went well over, possibly by the editors suggesting her to let go of those restrains.
I personally think that George loves doing world building, maybe even more than writing drama and sweating over it as if suddenly he would have lost the talent. I suspect that he has finished the book a couple of times, but like anyone of us, he cannot get over the edit and rewrites. To me, edit might cut 15 to 16 000 words from 100k, but the rewrites will bring it back, and in some cases take it over. If he does not have a limit for the Winters book, or even for the Springs, then it will take time before its out.
I too have a number of characters I'd like to write out in the course of one book, but I try my best to not go there, and instead of putting them all in one, I compress things around the story.