Excerpts from an interview with GRRM which might explain this a little. Also, bear in mind, he's spent the last 20 years or so working with these characters. Every writer needs to work on something new now and then or they'll come to resent the hell out of the characters and the writing will suffer.
There's a danger of doing only one project, to the exclusion of everything else, and getting stuck in a rut or whatever.
There has to be a level of joy of what you're doing.
You actually said once you don't enjoy writing, you enjoy having written.
Yeah. Which is not original again with me either. A lot of writers have said that. But writing is hard. I mean I sit there and work at it.
Boy, there are days where I get up and say "Where the hell did my talent go? Look at this crap that I'm producing here. This is terrible. Look, I wrote this yesterday. I hate this, I hate this." And I can see a scene in my head, and when I try to get it down in words on paper, the words are clunky, the scene is not coming across right. So frustrating. And there are days where it keeps flowing. Open the floodgates, and there it is. Pages and pages coming. Where the hell does this all come from? I don't know.
I had, very early in my career, even before I was a professional writer — I'm going back now to my fanzine days in the 60s and 70s — I was very prone to starting stories and never finishing. I'd have some great idea and I would start a story, and I'd write a few pages, five pages, ten pages, and it would never be as good as when it was in my head. It was this incredible thing, I put it on paper, and it was never as good as I imagined it to be. Then I'd think of some other idea, and I'd go, "Yeah, that one would be really magical." And I'd put aside the half-finished one.
One of the big breakthroughs, I think for me, was reading Robert A. Heinlein's four rules of writing, one of which was, "You must finish what you write." I never had any problem with the first one, "You must write" — I was writing since I was a kid. But I never finished what I was writing. [I realized] I gotta actually finish these stories. It does me no good to have this drawer full of fragments. And always be chasing the next idea, which is so much better, so much more beautiful, so much more entrancing then the idea that you're actually working on.
So, I started finishing things. And I'm bound and determined to finish Ice and Fire.
The whole interview is here and I highly recommend it. It's got some pretty interesting stuff in it. No spoilers that I can think of off-hand.
George R.R. Martin: The Complete Unedited Interview