Looking over Wikipedia:
"After
A Storm of Swords was completed in 2000, Martin began writing
A Dance with Dragons, the intended fourth volume which would pick up the story five years after the previous volume. Martin found it difficult to make this work without an over-reliance on flashbacks. At the
World Science Fiction Convention in
Philadelphia on 1 September 2001, Martin announced that he was scrapping more than a year's work and writing a different fourth book that would fill in the gap, named
A Feast for Crows. He found it extremely difficult to go back and start again, especially as this novel was not planned for in his scheme for the series, and work on the book progressed slowly.
y May 2005,
A Feast for Crows had become longer than
A Storm of Swords (the manuscript for which had been 1521 pages) and Martin's publishers said they could not publish the book in one volume.
[16] They suggested splitting the book in two and releasing the volumes as
A Feast for Crows, Volume I and
A Feast for Crows, Volume II, but Martin was unhappy with this idea. After discussing the matter with his publishers and his friend and fellow writer
Daniel Abraham, Martin decided to split the book by character and location instead. The published
A Feast for Crows thus contained all of the characters in the South of the Seven Kingdoms, while
A Dance with Dragons contains the characters in the North, the Free Cities and in Slaver's Bay."
No wonder it took so long. There were two books there he never intended to even write, and probably by dividing it geographically instead of chronologically made it so he had to fit things for the 5th book to the 4th rather than just continuing along and having the freedom to change some things if needed from effects happening concurrently elsewhere.
I imagine the next two books will go along more quickly and smoothly for him to write since he'll be going to back to the usual structure. Apparently he's already written 100 pages of book 6. So hopefully it won't be another incredible wait.