It takes so long, because GRRM vision of the story is so detailed and long...... and the fact that he tell so much of the story....
SOFAI was meant to be a trilogy..... it is currently looking to be 7 books.... but who know.
The next book should be released quicker than the 6 years it took for Dances as he has a good amount already written (ie, chapters that were planned for Dances, but pushed to the next book due to Dances getting too long)
I love the story, so I'm willing to wait so that it can be told in the best way.